I mean, how would they know something was stolen from a claim they've never been to before?
well, imagine having some rusty sword for sale on your stand, so old and rusty - nobody ever was interested in buying it; over the time the dust has settled in.. and the wood not occupied by the sword had became lighter due to sun and wearing off ; so once stolen.. you will see a footprint of the sword or even the silhouete .. left on wood ; you know there would be deductive and inductive methods of coming to a conclusion of what an item was there for sale and how was it taken. For example, the stand sells weapons, there are all but swords offered, so swords might have been bought out or missing? Or.. the stand of a trader famous for dealing in particular goods, one good missing -either bought already (which can be checked if the other goods are still there and if there are scents or tracks of the owner being hear and re-stocking recently) etc..
I am not the one into criminology, but there are ways Rangers (detectives) would be able to determine or narrow down the item, etc; provided we assume raising the skill their expertise in these area raises as well.