Bitmit is a legit service, individual sellers on Bitmit may or may not be. Caveat emptor, as always. But given the escrow protection offered by Bitmit, it is not a very risky proposition to make a purchase even from a new seller, and it all seems to work very well. The owner of Bitmit recently commented as follows at Reddit:
Last month we changed the policy to an escrow force for new sellers. (In fact almost all offers on Bitmit are with escrow now). That works very very well. Before that date we had a few issues with scammers who sold without escrow & naive buyers.
Successful fraudulent transactions with escrow are almost against zero.
There is no single order on which the funds of a escrow transaction have not been released to the seller or buyer. There are only 2 cases we really needed to arbitrate to come to a solution. We can conclude that scammers are only a very tiny problem on Bitmit.