Either get a vanilla account (they cost 4 euro.) and trade through that (it will get banned.. in general doing f2f trades will get you banned..)
use a guildbank, invite them and have the amount of gold they've bought set as the maximum withdrawable amount pr. day.. let them grab the gold, and kick him.
Or! trade through the AH, it takes 5% (?) of the total gold, but it's the safest method imo. i do that to "white-wash" my gold from my botting toons to my main acc.