Well, I'm new to this steam cards business.
But I get a few cards coming in these days since I started a farming software AFS to get these pesky cards out of my games without me needing to do it. So I can install and play what I want to play and just not worry about cards.
Well what I did until now is sell every card I get on the market for the lowest price it's currently sold for instantly. (got 7,5€ store credit in the last three days that way)

Seeing this thread, I wonder if that's the right thing to do?
Is it worth it, collecting whole sets steamwallet wise? because that's the only thing I care for, so I can save a few bucks when buying moar games ;) I do not care for upgrading my steam profile or bragging with complete sets. Only more money for more games. Is it sensible for me to try and hold onto my cards and trade and try to complete sets?

No. Since you cannot sell complete sets (which in physical collectible like stamps or cards usually go for a higher price), the only thing you can possibly get from completing badges is a slightly higher chance of getting booster packs. Every 5 Steam levels, your chances of getting one increase by 20%. So in your case, selling the cards is most likely better.
Do be aware that selling over a certain amount worth of cards you will need to fill an income form, and you may also need to do that if you sell over 200 items or something.

thanks :) So this booster pack that's just 3 cards that I can again sell, but I probably won't get more cards from booster packs than the amount I have to "spend" for leveling up my account.
So I'll keep on just selling the things and hope it'll soon end (farmed 'em all) because the amount of time I have to invest to sell these critters isn't worth the few cents I get :/ Still it's nice to have a discount for the next purchase ;) ... but I really don't get the people buying the things... well, each to their own I guess.
(I'm sure there's enough people, too, that'd look at my account and say: "Why is he buying all those games? I don't get it ;)"

yep been there for the 200 items ;) and I didn't like it. why fill out US tax papers when I'm clearly not a US citizen (which they ask about in EVERY purchase) and they even sent me a steam controller so they know my address and know that I'm not from the US. oh well. no data they didn't have yet, so I just filled out the form ;)
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So anyone interested in ASF? it's "advertised" by the dev here

and what I really love is how easy (in the end) it was to set it up in a docker container and let it run on my nas (so it doesn't cost me any energy and no computer has to run that doesn't run anyway all the time): here's a tiny howto I posted also in the above thread:

so there's a mono docker, so easy as this I got it running on my nas:

1. mono ASF.exe #find out if everything works tweak config, log in
2. create dockerfile in the same folder you have ASF.exe and your config folder in:

FROM mono:latest
ADD . /home/someplace/asf
CMD [ "mono", "/home/someplace/asf/ASF.exe" ]

3. sudo docker build -t ASFdocker . #build the container
4. sudo docker run ASFdocker #run the container to test
5. sudo docker save -o ~/ASFdocker.image ASFdocker #save the container to a file to upload to nas and start there.
6. Profit!

EDIT: I should probably mention what ASF is. So it's just a small app using the steam api to tell steam you are playing a given game and checks your badge progress until it farmed all cards in one game and it plays the next. So around 2 hours each game until all cards dropped. it runs in linux, mac and windows.

It's been running without hiccups around 70 hours on my nas now and has farmed me more than 200 cards in that time. It does not interfere with my normal usage of steam, when I want to play a game it just stops farming and starts again when I stop playing.

Wait a shotgun cocking minute, I can farm cards without playing my games? 0_O

WHY DID NO ONE TOLD ME THAT BEFORE?

hehe, yes. And it saves energy aswell... So kind of a green thing to do ;)
but of course you can still play your games as you please, it's only you don't have to just to get the cards out of 'em ....
now if only I had a bot to sell the cards, too. that'd be nice....

EDIT: ok looked it up now. the ASF is running one week now (no restarts or other tweaks needed after initial config it just does what it's supposed to.) and I got around 300 cards since then (160 still in market around 140 already sold for around 10€ steam credit which I then blew on moar games ;)

EDIT2: changed the number from 450 cards to 300 cards since the market history lists each game twice. once for putting it in the market and the second time if it's sold....

Every 10 levels, no 5 levels - https://steamcommunity.com/tradingcards/faq

Thank you for the correction, was going from memory.

Short update: ASF is farming two weeks now without any problem. it's impressive how stable that thing runs.
In theese two weeks it farmed me around 500 cards from which around 360 are sold and netted around 25€ store credit. now if only I had started this before the summer sale ;)
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What are foils btw. ? really just more valuable Trading Cards, or do they serve any purpose?

EDIT: btw, is there anyway to tell steam, that one doesn't want to get a mail for every single trading card sold? it's the most spam I've ever gotten. Right now I'm archiving the mails to get rid of them, but I guess I'll soon get the spam filter to sort them out...

Yes and you can craft only 1 level badge from foil cards.

thanks, for trying to help :) but alas there isn't any button :/ Steam views these as a reciept for the transaction that took place. but when you get 25 of these each day, it's a bit tiring. Well, I guess the spam filter will have to work it out.

Oh. Well, some email accounts (e.g. yahoo, google, etc.) have a way to make "rules" to move received email immediately to a specific folder, or some other actions. Or if you're using an email application then probably in the Preferences for it there would be a way to make a rule. Probably match the email address or something in the subject line, or both to make sure it's a proper match. With rules your incoming email would be automatically sorted before you check your email. :-)

I realize that's probably what you mean to do with spam approach, but in this way you could archive them separately without affecting other users that don't want such emails marked as spam for themselves. I've gotten the impression email services tend to try to learn from what their users mark as spam and then apply that to sifting out spam from everyone's incoming email. If you really don't want to keep them in a new separate folder, then instead you could have a rule to just move them to the trash instead of marked as spam.

That might also help ensure future receipts that do matter, perhaps from other companies, are less likely to get marked as spam.

Valid point, I hadn't considered this. thanks. I'll use the label feature of gmail, I guess :)
EDIT: aaand done. filtered easy as that. :) no more steam market mails. aah, the silence from the phone ;)