From my point of view, the value of the probation list is twofold. Firstly it shows potential buyers or sellers who has a record for being bad to deal with and therefore protects the potential buyers and sellers. Secondly it is a significant incentive for anyone buying or selling to conduct themselves in an honest and honourable manner, basically honour your deals or you'll be put on the probation list and it'll stop people from dealing with you.
RMA has welcomed probation listers so is obviously not worried about being ripped off by anyone which is his potential problem. By stating that he welcomes probation listers though, he's deliberately giving those who have been put on the list for reneging on a deal, a way to buy here and therefore lessen the effectiveness of the probation deterrent.
There aren't any rules against dealing with probies so if RMA doesn't give a damn about those who have been messed around by those on the list, there's nothing to stop him from doing so. In my opinion, there should be something in the rules about it though so maybe it should be put to the boards to amend the rules?