The sites themselves are trustworthy. However, as with all Runescape accounts, they can always be recovered by the original owner. Most of accounts on said site are sold to them; there is no guarantee that one day the original seller/owner won't recover the account.
When purchasing accounts, it's best to buy it of someone whom you can fully trust. I'd personally say look into established sellers here on Sythe, in particular Video, KMJT and Snow Patrol.