I wouldn't buy from these types of sellers but they exist in just about every line of business. How many automotive service writers do you know that encourage unneeded work or sell those overpriced preventive maintenance packages when the manufacturer's service recommendations are mostly visual inspections and basic lubrication? In my eyes, those actions are thievery too. Buyers need to know what they need and what the going market price is for the item they want to buy.
When I'm in the market for something, I'll often check Ebay to see if there are any good deals but on lots of common items, I can get a better delivered price from Amazon or another reputable internet seller than going with an Ebay seller with overpriced shipping (although shipping is expensive, asking anything more than retail UPS/USPS/Fedex rates is robbery, with very few exceptions like special packaging requirements).