I can't really say whether it's a reasonable price, but I can tell you that you'll rarely find a Remington 870 of any breed, in decent shape, for less than that. Rusty and beat to hell, maybe; but not in very good to excellent. So if you want a used Remington pump in nice shape (unless you have the patience to wait for months/years, hoping to stumble on a great deal), that's probably pretty close to the best price you'll see.
Ask them if they guarantee that it will function safely and properly, and what your options will be if it doesn't (Money back? Repair? Replacement? etc.).