buy a used lens from Ebay

How to judge if it is a good deal? particularly how to tell the condition of the lens? Noticed that some lenses got so many bids some got few although it seems from the seller's description they were almost the same.

Stare at the pictures, as questions about its use. It's easy to hide nicks on the lens, so cosmetics is usually hard to discern. Performance wise, if the seller's honest, you should get a pretty good idea based on what it's been used for and how much it's been used.

For example, if you shoot seaside or on beaches, sea water can get on the lens, sand and salt gets left as residue. Or if you shoot events or wildlife, the lens can get pretty battered. The elements can be pretty harsh on a lens.

Hard to judge on eBay, although there are some truly genuine people on there. My suggestion would be to contact the seller and get as much info about said lens as possible.
I bought a 70-200 IS a couple weeks ago from Fredmiranda.com and got a deal, plus, felt more at ease being it was from a forum dedicated to photography, such as this one.
Chances are the lens you seek will be available here, but if not, check FM forums or like I said, try and build some sort of repor with the seller on eBay and get as much info on the lens, buy from a seller with 100% feedback and whom accepts returns in case you're unsatisfied.

My 2 cents...

I sell older lenses/camera on ebay all the time. Last thing I want is someone either returning it, or leaving me negative feedback. I post lots of clear solid images of the item, and try to describe at as best as I can, and honestly. I will look for scratches, dents, dings, fungus, haze, etc., and fully describe my findings. I look at what others put up as images or a description, and there is no way I would spend my money on a fuzzy/dark picture of a lens. If you don't know how to take a picture of a lens, you probably didn't know how to care for it. If the description is "It's a great lens", it probably isn't.

Pictures, pictures, and more pictures. You want to see every angle and corner of the lens. Ask questions, and watch the feedback.

My 70-200 f/4L and 300 f/4L IS were both purchased on Ebay, both in mint condition with 0 problems. The 70-200 was purchased from Henry's Camera(Canadian Store) so I didn't hesitate to trust what I was getting. It was used but listed in mint condition with a 30 day money back guarantee.

The 300 f/4L IS was purchased from a random Ebay seller. All I could go by were the provided pictures, answers to couple questions(seller responded within a couple hours and provided some extra hi-res pictures) and his 100% feedback. The rest is just blind trust that the seller is honest and knowing that Ebay/Paypal will get involved if the deal goes sour.