With my Mossberg smooth-bore I can get 6" groups at 100" using Federal rifled slugs.
Note that the barrel for sale on eBay is itself rifled... you don't really want to shoot "rifled slugs" through that... it won't hurt it, but it won't do your accuracy any good either. Shooting saboted rounds through either a Hastings or Remington rifled barrel should yield groups on the order of sub 3". By the way, that's a quality barrel and a fair deal.
Have fun, but be prepared for a shoulder battering when sighting in.
p.s. IMPORTANT: Unless your 870 receiver is designed for 3" magnums, DO NOT shoot 3" magnums in a barrel chambered for 3" magnums... suffice it to say, "you'll put yer eye out".