Keep providing your skilling/account sale services, if you get someone to use your services then ask them to vouch for you.
For account trading you need a good amount of trust imo. So what I would do is skilling services, but I would maybe do some kind of special offer for the first few customers. I'd also do skills which (dont require bank pins or aren't going to take weeks to do) because no one will trust a new-commer with all that.
In addition to Don's post, upgrade sales are also a good way to make vouches with little risk of scammers involved. Use MM's for your first few trades.
I think it is also acceptable to vouch for someone who has helped you out with something such as paypal, account recovery, computer related issues, and other tasks.
If you think you've did a lot to help someone or vice versa, it doesn't hurt to ask.