Too many people read too many stories of amazing finds at flea markets, garage sales, newspaper ads, etc. Everybody is afraid they'll flip through the channel one day, see the guy they sold their collection to on the 'antiques roadshow' just in time to catch the crawl at the bottom: 'vintage baseball cards, $20 craigslist sale, est value:$1500-2000'
People get defensive all the time, because too often the people trying to 'help' the seller ONLY do so if they are overpriced. If they were selling a NrMT 61T Mantle for $20, the buyer wouldn't very helpful, they'd hand them the $20, run back here and brag about the 'steal of the century'.