There might be a few out that would pay top dollar for a clean 500. They are the two wheel kings at the dunes. I'd keep it and get a WR.
+1 No reason to off a fully rebuilt KX5. You'd prolly get $2K on a good day for it which I suppose you could use for the WR but there's just more value in that KX5 than $2K. I would make it a dune bike or a snow bike or use as is for days you just want to roost your friends.