There's two issues.
One, as HM has mentioned, is that there is a potential can full o' proverbs involving RL monies - and we'd like to stay away from that as much as possible.
There is also an increased potential for fraud v Illy corp as well.
Currently, if a player chooses to buy prestige and spend it, and *then* chargeback (dispute) the transaction via the payment provider or credit card a few days down the line... well, they don't really get anything because we end up suspending their (and any related) game accounts pretty quickly. This is why this happens very rarely; as there's very little advantage to it. Players who try this can also end up in all sorts of knots with their payment providers (PayPal, GCheckout etc), which isn't worth the candle for £x of prestige.
But if they did the fraudulent chargeback route and had actually disbursed the purchased prestige to third party players, then things could get very, very messy indeed.
This is why appointing a sitter has so many very dire warnings attached to it. Sitters *are* treated by both the system and the GMs as though they were you personally, and if a sitter uses your account to trash it, or gets it suspended (or whatever) then I'm afraid that whilst it's unfortunate, it's also "final".
Any Petition variant based on "X player trashed my account whilst a) sitting it for me, or b) using my login credentials" gets a smidgeon of sympathy from us - but otherwise short shrift.
Regards,
SC