That's a good solid unit - there is currently an SM050 on eBay going for $450.00 and when I saw that I said "wow, good price". I BELIEVE the SM055 is an "updated" version of the SM050.
Really a solid no nonsense unit, 5 shelves, manual temperature control. Rocking for pork butts, brisket, ribs. $275.00 is a very good price. Really all you want to do is see that the door seals up tight when you close/lock it, and that when tured on it SMOKES. Call the seller before you go and ask him to fire it up with a small chunk of wood in it. You'll be able to safely unplug it and pick it up - the insulation keeps the outside of these things quite cool considering.
Good find !!