You can try selling them .. but, I know I would never buy one ... not second hand, that's for sure.
A person who buys a very important piece of safety equipment 2nd hand has no idea what condition it is in. Sometimes you can't tell if has suffered an impact (even by dropping it onto a hard surface). It might be as worthless as wearing a sock on your head...the only benefit is the legality of wearing a helmet in areas where it's required by law.
You can sell your used motorcycle helmets fast at online sites like eBay. Depending on what type of a helmet it is, you can sell them from $20 to $100. Look at the price you bought them and the condition they are in now to help you determine their worth. See more details at http://www.ebay.com/sch/i.html?_nkw=used+motorcycle+helmets.