Nothing wrong with a flip. Might be helping the guy support his own fish budget, or simply pay the mortgage. The bottom line is, once the seller agrees to a price and the product changes hands, it is no longer the seller's to have an opinion on. If the buyer wanted to fill it, sell it, or set it on fire it is his to do with as he pleases.
Personally, I like to sell things for cheap or give away for free to avoid the hassle of marketing, haggling for top dollar, or entertaining tire kickers. People who sell retail, and deal with the hassle earn their profit.
Just my .02