It's a wee bit ironic in my case, as they offered to the quid pro quo after I had help them out a couple of times already -- of course, with the friendship gone sour, I'd say that's no more the case.

It's important that you mention a written contract. Couple of times, these have been transactions out of the ToS due to the artists' circumstances and that they were in need of the money/goods/et al.