These types of queries are not complex to write at all, but just radio-button selectors on an interface.
Sure, a radio button would be a start. But because the search criteria order would matter then, you have to add functionality to reorder the criterias. Off the top of my head I can't come up with more, but I wouldn't be surprised if in practice more is needed to keep it usable.
Once the Vendor Search function goes live, I will bet that Petra will have a page explaining all functions and options.
There are tons of players out there, who don't even know, what stratics is. And who never visited nor will visit any forum. They just want to play the game. I know quite a few such players.
The stratics community is just a small fraction of the overall playerbase. Some players visit other forums, after all there are quite a few UO forums in various languages out there spread all over the world. And some don't visit any forum.
I don't see the as being hard at all to explain - I've had to do it before for very complicated Access UI's. The vendor search is comparatively simple.
From your point of view it may not seem hard, because you're already experienced with it. And you most probably wanted it to learn or you had to.
So I'm not talking about the intelligence of players, rather more about their readiness to learn something new. For instance, two frineds of mine just know the absolute basics about item properties. Every once in a while they come ask me, "ist that item better than mine?" etc. I gave them the links to several pages on stratics and uoguide, but they don't want to dig into this. They just want to play the game. They don't have maxed out suits, still they're quite happy with their current situation.
If you don't understand these logical operators, don't use them.
Let's stick with your radio button example. Assuming, that there is no additional optional simple UI, there will be these radio buttons. And either AND or OR has to be selected. So they would have to use them, wether they want or not.
The only feasible way, I can see this work, is to hardcode the operator to a certain type of search criteria.
For instance, every item property uses an AND. Just like it is currently on TC1.
Mutually exclusive search criterias use OR, if more are selected. Currently only two categories, namely "Equipment" and "Required skill" behave like that. If you select another criteria from the same category, it replaces your previous selection. This could be reworked, so that multiple selections within the same category use an OR.
I.e.: You need either a ring or a bracelet with LMC 5% and FC 1.
Would look like in the UI like this:
LMC 5%
FC1
Ring, Bracelet
Since OR is used, they're displayed in the same line, separated with a comma. Every single line itself would use AND. So the query itself would be:
LMC >= 5% AND FC >= 1 AND (Ring OR Bracelet)
That would make the whole thing more powerful but would not overload the UI with functionality, which most people most probably wouldn't use anyway.