technically, there is no law says the buyer can't do that,
it is the free market, sellers can price their products or services at a price they feel it is reasonable to him/her, and buyers can also take up the offer at price they feel it is reasonable to them.
anyway, the item already exchange hand and you have collected the money, so it does not belong to you any more, the buyer keeping it or selling it away is at his own free will, no longer your concern, likewise if you do the same with an item you bought from somebody, the previous owner has no right to say anything about you too.
however, if you suspect this buyer is a trader aka commercial seller, you can use the report post function to alert the Admins and Mods team. the personal classified section is for members to buy and sell photographic equipment on a personal basis.
hope this help.