Just like the stores here that sell stamps can not charge extra for them !!
When I worked at the Post Office, I would get this query all the time. Any store selling stamps can charge whatever they want UNLESS they are a licensed USPS branch, station, affiliate, or official USPS mailing center. A convenience/gas store can take a 44c stamp and charge 45c or 50c or whatever their customers will put up with. There is no law against that.
I agree on ebay not helping the small, honest sellers but they either do not care or are making tons of money from BIG sellers. There must be a mass exodus from big sellers to get ebay to do anything. Small sellers leaving do not hurt their bottom line. I would love to go somewhere else but there is no traffic comparable anywhere else right now. The other sites are fixed prices or social network sites.