The only countries who might be willing to do that (tinpot little dictatorships, presumably) are also the ones who are likely to claim it back after a couple of years. ("What? We never sold you this island!")
Better not bet on it :) There's a big difference between selling the legal ownership to land and giving up all sovereign rights to it.
I heard Greece could use some money
Germany actually suggested that Greece sell some island to pay their debt - more of snub than a real comment though I think
Even though, they'll still belong to Greece, it's the same as buying any other property. Just a bit more expensive.