I would ask two questions:
1. Would I buy from this seller?
2. Are the seller auctions competitive?
If the answer to the first question is no, is it because you don't trust them?
If the answer to the second question is yes, are prices realized often above what you would pay?
If the company sells on
ebay or the web, checking old web sites like
www.scads.org could be helpful. If a seller was mentioned in a bad light ten or fifteen years ago, has anything changed?
Also, does the seller offer uncertified stamps that that have no value without a certificate?