It's been said already, more or less, but my view is that at the moment each and every caravan is coming to your humble site just to trade with you, and apparently with no other ports of call before or after. If they end up leaving with something they brought, then that's wasted effort. If they're willing to discuss swapping silk cloth, magnificent pre-trained beasts or ingots of all the various metals (except one!) that you may well be without for some ☼masterful☼ stone earrings and whatnot, then that's their business. That's trade. Haves with a want meet have-nots with an abundance of something the latter consider worthless, swap their respective surpluses and each imagine themselves on top of the deal.
But as and when caravans are more accurately modelled to run a circuit consisting not only of your settlement but of numerous human hamlets, elevated elven encampments, slugman settlements, possibly even a dark tower or two, they're probably going to be a bit more picky. "Hey, Urist McTrader. I know your place makes good rings and all, but last year the Goblin Lord complained that we'd not got any caged Megabeasts left after we'd passed by your place, and after only a couple of annual circuits, all our other market-places are now peopled (and 'thing'ed!) entirely by various beings with so much jewellery on that they constantly rattle and occasionally even collapse to the ground and can't get up again! We'd take all your fine crafts, yes, but I really can't see us able to swap even one low quality donkey-cage (with one low quality donkey, in it!) for less then a thousand or so figurines. What? What do you mean you'll take five!?! Oh dear, my poor camels, I'm going to have to talk to my union representative about this!"
(Additionally, I've always wondered what it might take to get some of the traders or accompanying guards to relinquish some of their personal equipment. Perhaps rarely possible, but some guy might like a nice vegan roast in return for a silk cape, or a wagon-driver (if they ever happen again) might even trade his shoes for a meaty FB biscuit or two.)