Have not bought them through e-bay, but we found a great semi annual warehouse sale at Gingess, selling both used and new. We got the whole outfit, with 2 shirts, couple ties, vest, shoes for $300, so somthing else to check out. You are right though, much better just buying a tux than renting, just a few wears will cover the cost of a few rentals. If you plan on more cruising, its the thing to do.
Shopping on Ebay for a tux is great. Mine....shows NO wear, seems new...tho it might not be the absolute 'latest' in the tux world, will do wonderfully on board. I think it was $50.00, plus shipping. It had coat, pants vest, tie....had to buy a shirt and studs and shoes. Tis an 'After Six' tux....probly would have been 400-500 new. I am delighted.
As for Ebay, just be prudent...read feedback for each seller.
We haven't bought a tux on eBay, yet, but I have bought 2 evening gowns and am thrilled with them. We looked at the different sellers and found some that owned formal wear shops and sold their overstock on eBay or sold their discontinued floor models. I ended up with 2 (new with tags on) dresses for about $40 each (including shipping) when each dress retailed for over $200.
Check out the seller info. Make sure the feedback is good and then go for what you think is a good price.
We only go on a cruise every couple of years . Even though it is $85.00 to rent a tux and have it delivered to your room. It is so worth it as it is pressed and comes with 2 shirts and you don't have to pack it!!!! Also people do change sizes and then you will have a tux that doesn't fit. But good luck!!