Since introduction of limited carry weight for NPCs, there is an exploit with foreign traders. When you sell them a fur stack they can't carry, they will dump it, and you can pick it up and trade again, to this same guy or others. Normally it happens only to heaviest stack, or least expensive, but if you use only one stack, they dump it anyway, instead of splitting it, for example.
The best stacks would be elk and bear furs, with fine or superior quality, and winter variety. Because the stack needs to be quite big, it isn't exploitable by early characters, since it needs an investment in furs, pack animal (you need a pig at least), and optionally hideworking skill. Once you have so many furs, you probably don't need it, so it's more for fun of thievery than for anything else. The foreign traders came here to exploit you - you can exploit them instead!
The minimal usable stack seems to be 5 elk furs, but it's not safe, many traders are not encumbered enough to be unable to carry it, plus if you buy something heavy they are encumbered less. Stack of 6 furs is much more safe, still there are guys able to carry it. Stack of 7 can be carried only by exceptional strongmen, who need to be lightly armoured at most, and you probably won't meet them. I use stack of 8 to be sure they can't carry it. The bigger the stack the more you can buy, of course. Stack of 6 fine elk furs of winter variety (easily achievable with even mediocre skills) will buy you the most expensive weapon of masterwork quality, for example.
This exploit is not possible with villagers, because the things they drop are considered village property, even if dropped outside of village borders. It is possible with woodsmen of any sort encountered, though.