Resale lives or dies on trust, because every item is different and the buyer cannot hold it. Three things build that trust: honest condition grades, clear photos and a fair way to settle disagreements. You set the standard for all three, and the sooner you write it down the fewer arguments you will have to settle by hand.
Start with condition grades. Keep them short and concrete, such as new with tags, like new, good and worn, with a sentence for each that says what a seller may and may not call it. Show the grade near the price on every listing and make it a required field, since it is the one detail buyers always check.
Photos come next. Ask for a front, a back, a label and any flaw, and show those prompts in the listing form on a phone. A seller who skips the flaw photo is the seller who ends up in a dispute, so make that prompt hard to miss without turning the form into a chore.
For disputes, publish a simple process. A buyer reports a problem within a set window, the seller replies, and you decide when they cannot agree. Say who pays for return postage in each case. Having the rule written before the first dispute turns an argument into a lookup.
Finally, use the community to reinforce trust. Sellers who answer questions quickly and post honest detail in the feed build a following, and buyers follow the sellers they trust. Feature those sellers on the homepage and thank them in public.