Your payment is held until you confirm the account works. Confirm and the seller is paid straight away; say nothing and it releases after 5 days, so there is time to be sure the account is really yours. How escrow works
Roblox accounts are bought for three things: age, limiteds and username. An account made in 2010 cannot be made again, some limiteds are no longer obtainable at any price, and a short clean username is its own reason to buy.