I am happy to buy used, but rarely from Ebay. Usually I am buying on forums,.. here and at Fred Miranda. Usually from peo[ple I feel I "know" even if we've never met. I've bought and sold a number of multi-thousand dollar lenses at FM ad here.
I do buy new of course, but most of my big lens purchases have in fact been used. At SUBSTANTIAL Savings over new.
I did do one BIG lens purchase from Ebay however, it was a SIGMA 300-800mm f/5.6 EX HSM
I won't mention the price I got, 'cause it will make you all cry, but it was Brand new unopened stock, when the newer "DG" version had come out, so it was considered clearance, and the vendor had somehow ended up with 7 of this "old" version. I got the 7th one at the lowest price.
The sale was a perfect transaction, amazing price, and the lens was indeed in a factory unopened box.
There are a number of real vendors selling on Ebay whom I would trust, and if you look hard the deals do come up, but they are far and few IMHO. It takes real time to find the good and safe deals.
Another good source of used is KEH, B&H, and Adorama and Henry's in Canada to name a few. (many also have Ebay stores)
Here the prices aren't a steal, but you are dealing with a vendor that will take a return if you are unsatisfied.