He must be very skilled in item lore or magic, as he is the only one in the early Wizardry games who can uncurse a piece of equipment - which is then lost. Like a bishop, he can also identify unknown equipment. None of his services are free.
Once you have discovered the stark discrepancy between his selling and buying prices, you know that Boltac is running a monopoly. Only in Wizardry V, the heroes will encounter other traders in the maze, but their prices are truly a rip-off.
The Ultimate Wizardry Archives (Manual), p. 17.