A high number of iTraders and posts is a must, as is her/his grammar when posting, but no one is ever 100% trustworthy. And remember if something sounds too good to be true, it almost always is.
I have a criteria of trusting members having good number of itraders and 100% positive and looking at their threads having 100% positive reviews as well that look natural too... I don't care about paid or free members at all when dealing in BST section Thanks PHYza
iTraders are joke. I see people forcing the buyers to leave iTrader, by promising some extra stuff as bonus. Sure I am talking about members with 300+ iTraders. I see the quality of posts of a member to examine his nature. And working out a deal/payment in chunks is a good idea IMO. And yes, never buy ebooks, all are bound to be BS. No one would sell you his/her "working secrets".
I trust members with I trader and proportion to their in existence period in DP. That's i get an average view over their I trader value and total period they are member of DP and how many posts they are counted and see up their ITrader comments and check whether their ITrader givers are with good iTrader and also take risk if the deal is some few dollars, if it is big amount then i prefer to ask upfront or review.
I trust on that people from those i have already done some deals and care when deal with a new person !