When I go into the Adventurer's Mart in in Waukeen's Promenade and try to steal from Ribald Baterman, the opinion is just not available.

With Lady Yuth, sometimes the option is there and some times it is not.

What does govern whether you are allowed to try stealing?

Any ideas welcome!

If I remember correctly there's a guard there warning you that you shouldn't try anything funny. While you can't steal anything there you can attempt to pickpocket Ribald. Doesn't give you much, a ring was it? but you can try.

A common theme is where there's lots of valuable stuff and guards around you aren't able to steal. At least that is how I remember it being in the original BG2 Saga. Never played Beamdog's EE so things might be different there.

Some stores are flagged as 'stealable' under the hood and some aren't. Not sure why one would be 'sometimes'. Each store also has a difficulty rating for the stealing.
You can actually change these using Near Infinity or DLTCEP but it won't affect a game in progress after you've visited the store for the first time.

As said above, there's generally an in-world reason as to why you can't steal. For other things, the game makers probably wanted to avoid you cheaply getting some items early in the game.

The Adventurer's Mart is flagged as "Not Stealable".

As for Lady Yuth or other stealable shops, the Steal option is only there if the character interacting with the shop is a thief.

Thanks for your helpful replies; I had overlooked not trying to steal with a non-thief! It is also good to know that the Adventurer's Mart is 'not stealable'!
Thanks

Ring of regeneration. It's a decent item to get early in the game. I think it might also reduce rest time (to the base of 8 hours), which is handy if you are trying to speedrun or solo the game. I can't remember for sure, and would have to test it later on.