EBay, Collectors, Flea Market & Antiques

As other posters mentioned, selling costs on eBay add a great deal to your overhead. We sold antique and vintage items until 2016. As our effort evolved, we sold small items that could fit in a padded envelope or 6X6 box. If we sold, we had buyers pay shipping, and if we bought, looked for sellers who offered free shipping. Way of the world. If you offer free shipping, you'll have to add some or all of the cost to the mug. If the buyer pays shipping, you might not be their first choice. Be sure to check the MUGS>Sold and Completed filter and see what sells and for how much.

You might also try Etsy, or one of the other sites that sell artisan items? Maybe take orders?

We sold a few things on Facebook Marketplace, which seems to have the same reach as eBay and a lot less red tape. We sold a couple of huge lamps, which had to be shipped. The seller paid the shipping and we sent them via UPS. It was a good transaction. When we sold locally, we met people in public places, and that too went well.

There are also local sellings meets online. If you are part of one, you sell your items online, and then meet in a public place with other members of the group, and the buyers come and pick up their stuff. We did that one time.

You could also make a supply of mugs and set up at local craft shows, and also take orders from attendees.

Good luck!