Looks to be in decent condition. Wonder what kind of "upgrades" we're talking about. If it runs anything like it should, or at all, the money is certainly right.
First thing that piqued my interest, however, was that the graphics don't look like 1999 to me:
RS50s pics by year
Soooo, seller's forgotten what year the bike is, the bike's reg is incorrect (error seems to be in the wrong direction for that), the bike is wearing different plastic than it was born with, or... ???
But let's don't stand on ceremony, chaps: at that price, "OBO" to boot, it's time to bring out the wallet.