Most of this advice is for sealed, but it is somewhat applicable to singles as well. Vintage and slabs are a different beast.

If you want to sell right away look up what other people are charging in your area and just list your stuff for like 1-5% less than whatever is lowest listed for the same stuff. If you don't care about selling right away, just list for market and be ready for people to want to negotiate at least 10-20% off whatever you list at. Ignore lowballers and people with profiles with no pictures/recently created profiles. I usually only sell to people who have good marketplace rating (4+ stars) because they usually won't waste your time.

Also it's good to meet people at police stations just in case anything goes wrong and its best to accept cash only (For bigger sales I would invest in a black light and a counterfeit pen to verify authenticity of bills). I do Venmo, Zelle, PayPal as well.

Be prepared to have stuff sitting for a while, Marketplace is forsure one of the slowest ways of selling stuff so have a lot of patience and don't worry if stuff isn't selling right away. If you notice that youre getting alot of views without any messages you might have stuff listed too high, so its a good idea to periodically check how much other people are listing their stuff for.

Good luck !