It depends on too many factors for there to be one website with "average" prices. Location, quality, brand, equipment specifics, shipping costs, etc. There are quite a few websites that come up quickly with a google search, all of whom will ship equipment for an additional price. So you might find equipment for one price in Florida but shipping added will make it feasible to buy a higher priced item closer to home. (If shipping is listed as "free", the cost is built in to the price)
There are also auction houses who specialize in liquidating closed restaurants. Auctionzip.com lists auctions of every type nationwide but you can narrow the search by distance and type. I would also check the local newspaper and craigslist for your area. A bit of research should help you narrow down a typical price range for whatever you are looking for. Personally I would not buy anything I could not inspect first, no matter how cheap it was.
Not knowing where you are located makes answering you with specifics more challenging but if you are in or near a metro area, there should be an equipment distributor near by. As with everything, sometimes it's who you know. Frequenting those places and getting to know the owners/employees gets you in on the grapevine. Used restaurant equipment business is a funny one. Some operate off the radar out of a private warehouse, others have used equipment they haven't gotten ready for sale and there may be equipment not yet on the market someone can tell you about. Of course, establishing relationships like that sometimes requires the use of some of your spare time over the course of months and years but sometimes you just have to ask.
If you're friendly with other chefs/owners, ask where they shop. You can develop your own grapevine too.
Used equipment varies in value because of the condition it may be in. I would never buy without inspecting it first. I would be very cautious about any cooling or HVAC equipment. At auction or equipment bought from someone unknown I would estimate 10-20% on the dollar. I miss a lot of bids but when I do get one it is usually a good deal. If I need something such as furniture I may go up a little.
It depends on many factors. Does the seller have to sell? Yes=worth less, no =worth a little more. Can you remove equipment without damage to premises? Make sure the seller does not have liens on equipment. Get model numbers, serial numbers, and specs on equipment and get on the internet and do some research.