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.
The buyer and seller now use Linden Lab's Support Portal to create matching tickets, specifying the island and the agreed upon amount. The purchaser is charged through their Second Life account, and the seller receives the money in theirs, less the US$100 transfer fee. That does limit your payment options somewhat, but provides more assurance that you're the one who is actually going to get it, for the agreed-upon amount.
It is possible, of course, to specify a lower sum in the support tickets and handle the balance directly between you and the seller - though that part of the transaction would still be at risk if either party was intent on bilking the other.
On the whole, user reactions to this announced change seem to be very positive.