I’ve been looking into contactless card readers for selling merchandise at gigs. There seem to be two main ones – Zettle (owned by PayPal) and Square (owned by Jack Dorsey of Twitter). They both work with apps on your phone where you programme the price you want to charge (on your phone) and the reader connects via bluetooth and your customers can pay with their credit / debit cards etc. Both readers have deals on at the moment and can be purchased for under £30.
Check them out here