As with anything on Ebay, you have to be an educated shopper. If you've booked a cruise before, you know what the added expenses should be (port charges, taxes, etc.) all those extra add on fees. Read the fine print and know what you're paying for - a lot of TA's sell cruises on ebay to get business for their agency . . . . just don't bid in haste - look at what your buying, price elsewhere, look at the sellers feedback and ask all questions before hand.
Should be fine if you do your homework and maybe you'll get a really good deal too.