It’s difficult when offering print-on-demand services. I think the main photo should be of an actual item, but then mock ups of what an item could look like with, say, specific wording - but state in the visual that it is a mocked-up image, which would be useful to a buyer. From a buyer’s point of view, if it is for something bespoke, then a mock-up is essential, although concede that this could be done by email (or whatever) to the buyer.
Having previously worked for a printers where I was involved in the dye sublimation process (printing onto mugs, photo panels, t-shirts, etc.), I can say that it certainly didn’t feel like a mass-produced effort, especially if a customer wanted one mug. I would have to play around with the image in a software programme to get the correct size, brightness and colour; print (in mirror image) onto special paper; cut the image to size and fit around the mug; heat the mug press and place the mug into the press; cool down the printed mug in water; wash and dry the mug; reprint again if I wasn’t happy with the colour or image transfer (which often happened); pack and post the mug. Felt like a hand-made process to me!