Despite being able to sell plenty of them, the only spells Tahm can actually use are Open Very Easy Lock and Minor Fortify Security. He has a house in the Elven Gardens District. Additionally, he provides training in Security up to level 75. 1200 gold is the maximum gold available from any merchant - Varel Morvayn is a Dark Elf smith and merchant who owns Morvayn's Peacemakers in Anvil and 1500 gold is the maximum from the highest level fence Fathis Ules - Dunmer Thieves' Guild fence in the Imperial City. The Restoration spells he sells are also unique. He sells the unique Potions of Magnification and standard Open scrolls.
This is where Tahm spends most of his time (8am to 8pm) standing and reading, in the northwest corner of the main cave (near the entrance from Smuggler's Cave). The related upgrade also includes a 'Training Chest' that can be used to effectively train your Security skill: it has a Hard Lock that immediately locks again every time it is successfully opened. He is armed with a fine steel cutlass and carries a copy of the Guide to Anvil and some crab meat. He wears a brown shirt, light brown linens and a pair of rough leather shoes. Tahm Blackwell is an Imperial rogue who is included with the 'Security Expert' upgrade that you can purchase for 1000 gold from Dahlia Rackham for Dunbarrow Cove.