Most BDSM Slave Auctions sell slaves/submissives for L$. There is a set contract with limitations the slaves/submissives themselves lay down before they actually go up for auction. Most have some expectations of RL interactions whether by voice, cam, IM or otherwise but some are just looking for the roleplay aspect as well. It is up to both the slave/submissive and the dominants/bidders what they are looking for in a BDSM partner and ASK questions!
I run a slave auction in SL and have been in the business for the better part of 4 years. I have seen many happy unions and many unhappy ones. For the most part it can be a very rewarding experience for all involved. It is a place they can go to find like minded people, so in essence what you are really paying for is a BDSM matchmaking service.