Well if you buy a account for RSgp you can trade it throught bh by let yourself get killed by the seller with worthful items on you. If he gets all the gp he'll give you account information such as pass, username, recoves ect.
If you sell your account for real cash you'll need a paymethode such as paypal, moneybookers, wu ect. The buyer sends money to the sellers account and seller will give account details.
You can also you a middleman wich is a trusted person in this community so you'll be for sure you won't get scammed.
Hope I answered your question good enought, if it's not clear just post it :d