I might be out of the loop, but are agents actually out there buying stuff from "unwilling" vendors at any significant scale? I thought that was still mostly limited to opt-in partnerships with retailers. Still, eBay might be anticipating the issues you mentioned and trying to get ahead of them.

Not commonly known (I work in this space), but yes.

Agents are being used to automate things like non-cash account balance arbitrage, stacking and abusing marketing promotions, triangulated purchasing schemes, and purchase-refund arbitrage schemes at an increasingly large scale.