One big issue I've noticed with it is that some items you'd think were of massive tradable value, actually have very low values.

Entire planets is one such thing. I've had CPU opponents refuse to trade me techs such as "Enhanced Miniturization" for even several entire planets. Also planets do not even appear on the opponent's trading items. You cannot buy planets at all. So, that event with minor races, the one that goes "our spies working closely with the <xxxxxxx> has stolen all of the technologies from <yyyyyy>, and are willing to trade. The <xxxxxx> are particularly interested in obtaining star systems from us". I offer them such and they won't even give out "Beam Defense Theory" for 'em!
Broken!

Ship trading seems busted, they usually refuse outright..... but if they do show willing, it's ridiculous prices, ie they'll only buy a Dreadnought off you for 2bc, but demand 20000bc when selling you a scout!
Proper ship trading is something I would really like to see in the game!

As for starbases. Trading in these just DOES NOT HAPPEN under ANY circumstances - even though they do appear on the trade item lists as if they can be traded.

I've had some races buy say for example, "Lasers I" of me for 2000bc, when another won't barely give me 20bc it!

The whole thing seems rather broken, and if I remember right, GalCiv 1 was the same, so this issue has persisted for a good dozen years. Rounds of applause for Stardick y'all! Whoops! I made a Fruedian slip!

It's not broken by itself, but there are multiple factors coming together that make it appear that way. For example, in the game files, the value for some techs is set so high, that the AI is only willing to sell them, if you give them everything you have, or refuse outright. Depending on the AI's plans, that value might be further adjusted (up or down).

In theory, planets should be valued pretty highly, and a fully developed one is usually valued as such by the AI (less so, if the planetary expenses are higher than the income). However, a freshly colonised world, or a recently conquered one with a very low population, is barely worth anything to the AI, regardless of the potential it holds. That's also one of the reasons why the CU removed the ability to buy planets off the AI. Something I don't agree with, but I wasn't part of the team anymore when that decision was made. The AI takes into account it's plans and the amount of ships it has, when making such a trade. For example, if it still wants to explore the map and you try to buy its last scout ship, then of course it's going to demand a ridicilous sum. Ditto when it believes to have a powerful military, and you try to sell it another ship. It's not going to give you much. If you want to play as a weapons dealer, then it is better to wait until (or cause) the AI to declare war on somebody else first. Once the AI starts to lose ships, you can get a much better price for yours. From both sides of the war. However, don't sell them your best ships. Always keep them for yourself. I have been able to buy starbases occasionly. Especially when playing the Terrans. But yes, starbases are one of the hardest items to buy. The most likely reason for this is, that the race giving you 2000bc had a ton of money, but no weapons and felt threatened. While the one giving you only 20bc had barely any money and/or already had a strong military.

Of course, your Diplomacy rating compared to that of the AI factors into all of this. The higher your rating, the more money you'll be able to make.