This is a real pet peeve of mine, as you know damn well at some stage the players/celebs will quite rightly just say enoughs enough and stop signing stuff...and real fans, kids etc will be the losers in this.
It's not illegal though, just a scummy profiteering thing to do. What's the answer? Maybe limit the amount of stuff people can asked to be signed- Won't stop it, but should lessen it slightly- And stop utter parasitic arseholes hogging the players to get them to sign a dozen cards or whatever, and let some other fans get a fair go too. The more true fans that can get something signed, the less will need to buy signed stuff off the scalping pricks.