Hmm.
I personally chose based on the status of my wallet. Also, if I buy 2nd hand, it's either through people I know personally or if I get clear pictures of the item, and even then I would always ASK to try before I buy.
However, no one way is the best. A very good example is the time I bought a 2nd hand Epi Firebird, inclusive of hardcase from a fellow softie. The deal went smoothly, but I didn't realise how big the hardcase was, and the sheer weight of it even by itself. Celarly I was out of my own reckoning.
The bottome line is, make sure you do your homework e.g. read any available reviews on harmony central, ultimateguitar, TGP etc; go to shop or borrow frm friend to try. Base your decisions on what you need, not just what you want...
many a time I bought blindly only to sell or trade off... very good examples for me would be a Boss OS-2, an MXR Distortion + and EHX Double Muff (notice how the bombastic names fooled me?) the former two having sold off and the DM traded for a Guvnor + respectively.
Sometimes you have to weigh the risks of purchasing 2nd hand... as can be seen from many a post here on soft, some sellers base your decision to make the purchase as a license to basically remove themselves completely from the item post-sale. Who can blame them from thinking otherwise, because in theory when you make the buy you have already taken the plunge to take on impications arising from the purchase, unless there have been prior negotiations.
Last of all... you get what you pay for, sometimes you get lucky, sometimes you don't. How to avoid bad deals? Unfortunately, unless you have very good connections, you just gotta ride them out and learn from your own mistakes.