If you had gotten a CTR on the low end of reasonable, I'd suggest optimizing your copy. Since you didn't, my impression it's one of two things: 1) people who liked ebay != ebay sellers and/or 2) facebook ads aren't very effective for this market

Try a more sophisticated targeting on facebook (can you target by people who have posted a link to an ebay item?)

If that doesn't work, I'd try a totally different channel. Seems like there should be lots of forums, blogs, and websites where ebay sellers congregate. Shoot for those.