You need to make good trades, and people will give you trust if they want. If you are higher rank this task become easy for you, and if you use escrow people will buy often.
this is the only we to become a "Trusted" seller. but there is no any way to become a verified seller,.
what I feel after reading your message is "You need a verified tag infront of your username by the forum admin"
if yes then this is not possbile. no one verified here officially by forum. everyone should earn the trust them selves.
i think that is exactly what OP want. I have other suggestion on how you show that your product is legit. i saw this method in the auction section that sells physical coin.
you just simply put a name sign in the product that you are going to sell and put a date on it. so that customer can verify that your product is legit.
other techniques is in the marketplace section. you can browse that