Physical location
Years in Business
Where they are registered
Phone #
References
Any other relative information
Most importantly: PAYMENT OPTIONS. Secure sellers typically offer secure payment methods. Look for sellers who accept credit card, L/C (letter of credit), escrow (with a reliable service), or fly and buy.
If you are planning on making a large purchase, plan on flying out to meet them. This will scare off most scammers and the small amount spent on a plane ticket is worth it

.