It's been said before (maybe in this thread, I'm not scrolling up to check) but the one additional tweak to the current system of trade demands might be to deprecate oft-traded items[1]. Just to keep the player on their toes.
A trade-advantage will not be offered for exactly the kind of thing you're already offering. If it looks like it might (based on some tally of recent sales) then it adds something to it. Bolts? No, now they would like steel bolts, etc. Not sure how to better qualify some things (like seeds) except going more specific to variety (which could make it even more inaccessible to take advantage of than with the usual lack of steel issues).
I take advantage of craft-and-dumpsell as much as anybody. Whatever stone type(s) I have plenty of that aren't useful (ore, flux, magmasafe) or aesthetic (a useful hue for building purposes) tends to be shifted into a Crafts industry that is almost entirely destined for trade (maybe also mugs) suitable for all visitors without ever worrying about elven wood-issues. If the Dwarven trades say that they'd appreciate rings then I'll hold back the rings for them, over the intervening year (and prob. buy in any and all rings offered by the intermediate caravans). I just think that there should be a limit. Maybe your site could become the actual Ring Capital Of The World, but (see footnote) that does not mean that it can pass on any old trash, year after year after year, and also expect that occasionally that some of the most artless forms of that trash is in peculiar demand.
[1] Or deprecate the below-average-quality items. Once you're turning out mugs in their millions, they become sought after if they are unusually good exemplars of the art. The cheap, tacky ones (or those 'merely' Masterwork-but-undecorated ones, once you've flooded the market) are considered kitsch, and now you must develop new variations every season to satisfy the outside world.