How Do You Research Trusted Sellers for Valuable Items?

I have been getting more interested in collecting items that hold long-term value, and I was wondering how other members decide whether a seller is trustworthy. There are many online stores and private sellers, so checking reviews, reputation, and customer experiences seems important. I recently started looking into options like a Bullion Dealer for precious metal purchases, but I would like to hear how others evaluate sellers before making a decision.