If the seller has overpriced an item, it's usually hard to offer them advice, but here are some ideas on how to do it with a minimum of offense.
1. If you are suggesting that he reconsider his asking price, its always good to send him a link to an expired auction to make your point. This is just a courtesy, since you are asking him to do more work. Another way to get price/listing information is from http://www.craiglook.com.
2. If you are not going to bid, and are just armchair quarterbacking, tell him that you unfortunately cannot bid, because you have enough instances of the item, but wanted to offer him some pricing information, just in case the item does not receive a bid. Remember that ebay always gives the seller a second try for free.
3. If it's someone who really doesn't know what the item is, "This Carlton International belonged to my Husband and now I can't bear to look at it any more", then I email them and try to tell them what a reasonable starting price and expected final price is (although I had no idea that MAFAC brakes were rare and valuable *sigh* on that recent carlton international craigslist / ebay sale.)
4. Always end your email with something like, "Best wishes for a successful auction!"
Personally, I only send messages about pricing to a seller when I'm certain that I won't be bidding. I sometimes send messages to the seller in the form of, "If you would consider shipping it by parcel post or surface mail, i can offer a higher bid", when I am bidding, but that's as far as it goes.
- Don Gillies
San Diego, CA, USA