For me, collecting sales tax on a private sale of used goods of any kind, goes against the grain. Never collected, nor paid, sales tax at a garage sale, a flea market, or in any transaction with an individual (as opposed to a store front or business like GB or GI). Items that required some sort of registration, like a vehicle, the state must be involved and therefore they are able to force a tax on the sale. Buying a used gun does not involve any requirement of registration, any more than buying a used pair of boots or a coat, therefore, the state has no business taxing the transaction, IMO. Guess all the FFLs I've dealt with here choose not to participate, as CO does collect sales tax; they must view it the same as I.