Definitely look at the reviews from regular users on BST threads. If the reviews are good throughout multiple thread pages, you're in pretty good shape(unless the BST thread OP decides to go crazy and scam all of a sudden. It happens, and it's strange).
I wouldn't buy from a new user's service unless I saw reviews from reputed members or the OP was known.
BUT, if I have profit I haven't invested yet, have no concrete investment plans for those funds(do not expand without a plan), and I'm comfortable to part with it all if I lose, then I always go for new member BSTs because finding that little gold nugget is so good. Not to mention the diversification it creates leading to more stable SEO.