Often pejorative; refers to someone who buys limited items (tickets, goods) to resell at a profit, especially online.
Scalpers bought up all the concert tickets.
Limited items are being listed at high prices by resellers.
ダフ屋 specifically refers to ticket scalpers, while 転売屋 can cover any type of reseller.
Compound of 転売 (resale) and 屋 (person/establishment).