Any craft item using seashells is an extremely niche market. I think they're more desirable for people living near the ocean or city dwellers wanting a beach decor. I would use your seashells to create other smaller craft items besides large mirrors (smaller mirrors, picture frames, photo album front covers, jewelry or whatever else you can create) in order to diversify your inventory. EBay gives you 250 free listings per month and I wouldn't pay for any extra gimmicks. I think craft items sell better on ETSY. I still have the bar soap dish I made out of 2 abalone shells (mount them back-to-back so one shell is the base and the other is the dish). Good luck!
