Auctioned merchandise

I am assuming the merchant decides what will be placed in the auction verses items they just have in their shop available for sale, is that correct? Also, is there a directory with all the merchants and a way to look at ALL the merchandise the merchant is selling. I have gone in a looked at the inventory of merchants selling in dash but how do I go back and pull their store? More variety of items need to be sold in auction. Same items over and over