my tips for how to judge trustworthy a site
1) view their page, look at what they describe about their service, such as xfer gold, if some site say they need 40+ def to deliver, its obviously scammer and they are outdated for a long time, also many scammer site appeal you with a SUPER cheap price.
2) talk to their live chat, if they are not pro and know nothing about Runescape, will you believe they will make gold for you?
3) search them on blackmarket site to see feedback from other player.
Hope i helped, btw I currently use www.bayoou.com most ppl dont heard of that but they are great