Some great posts here. Was away a few days and missed a lot.
Someone mentioned "flaw" vs "fault", point taken. I'm just a lazy typist (flaw has one less letter

) A "flaw" can in some cases be a good thing in a stamp (as in - a collectible variety)
The shipping issue has been addressed well, but my take on buying is this (apart from shopping around) -
- I include the cost of shipping in the price I am willing to pay for a given stamp (as if it does not exist as a separate thing).
So if there is a nice stamp I can get for $1, but shipping is $2.50, I ask if I would pay $3.50 for this stamp from a dealer in person.
If yes, then why not? If no, then I look elsewhere.
I really don't like to think in terms of "this seller charges too much shipping", especially as I buy from overseas as well as North America. Yes, they may be able to ship for less, but they need to make some money (after fees,etc) so I just look at the total price (combined shipping is something I take advantage of as much as possible as well.)
Regarding the sellers that use "Best Offer"; as a buyer I will make an offer if it is something I want. I won't insult the seller with something unreasonable, but as a rule of thumb, most accept 10% off the asking price automatically.
If your offer is more than a 10% discount off the asking price, they may accept or counter offer, just like in person. I have to want the item enough that I am willing to buy it in whatever the price range is. I wouldn't offer $20 for something listed at $50. I would look around and decide if I can get it cheaper elsewhere (ie. seller is overpriced) or make a "reasonable" offer.
Last: regarding the "specialist" price list. My thinking on this is to use research (shopping around) to get a feel for the market in any given area/at a given time. Specialized price-lists would become obsolete quickly.
For buyers, consider also that quality stamps (scarce, in very fine condition) have brought good prices (catalog and above) for a long time (even in poor economic times).