He carries a larger than normal supply of harpoons - 1000, which respawns 300 every 24 hours (one every 4 minutes 48 seconds), and provide an ample supply for Summoning needs. The shop also sells pet fish. Harry will sell you a tiny net for ten coins to catch a pet fish from his in-store pools, but you must have a fishbowl with water and seaweed in it in order to get the net from Harry (purchased through his dialogue rather than the shop stock).