Several times in the past, a very few greedy and disreputable Second Life users have acted to defraud would-be island purchasers by selling an island -- only to more than one person. Given that there was a delay in transfer of ownership for an island, it was possible for a dishonest owner to sell the same island to more than one person, but ownership could only really be transferred to one of them. One entrepreneurial user sold his island to six different people before vanishing with the cash. It certainly made things awfully uncertain.
Linden Lab's putting an end to that by requiring a new system for the transfer of island ownership, though it means that at least part of the funds transfer must be brokered through them.