As with any 2 cycle, remove the spark plug then the muffler from under the deck, blade and adapter have to come off first, and check the piston for scoring by looking through the exhaust port. You may need to remove the carbon from the ports first with a wood dowel, don't use a screwdriver. If the piston is galled or black it is due for an overhaul.
While you are at it, check the crank for side play and have someone slowly pull the rope. If the crank wobbles it is bent.
The commercial L/B's have grease fittings on the wheels so you can lube the bearings, a very nice feature. A mulch plate and side bag are available. The rear bag kit wont fit because of the fuel tank. Good luck, it is a good mower if it has been taken care of

Pay very little for it...it was used by 2 owners already, assuming one or two may have been a commercial lawn service, it may be tired out.

Thanks for the replies. The guy wants $75 for it. Sounds reasonable to me, but I have yet to see it in person.

We are assuming it's a 2-cycle, although the commercials are now 4-cycle and have been for two years. As you may know, the oil is mixed with the gas on the 2-cycle, so there will be some smoke. On a 2-cycle, the amount of smoke does not indicate engine condition. It simply means either it's running rich or there's too much oil in the gas. All Lawn-Boy accessories for 21" decks work on the commercial except the rear bag as already posted. It may have just a discharge plate in it, so you'd need a mulch plate. The bagging chute is green and replaces the mode plate (mulch or discharge). The blade is not changed for any mode of operation, and it uses the Tri-Cut blade. For mulching, you should install a mulch fan over the blade. 2-cycles don't have compression releases for two reasons; they have low compression to start with, and they have no valves to make the compression release work. Compression should feel pretty stout on pulling the starter, but not really hard. The engine should start on the first pull if properly primed/choked (2-cycles like to start 'wet'). It is normal for the engine to 'clank' when starting and stopping. It has roller bearings on the crank and rod which require greater clearance. The commercial is the ultimate Lawn-Boy because it's the utmost in simplicity. It just cuts grass. Where I used to work at a County government center, I used to watch the female jail birds use the Lawn-Boy commercials all summer long, year in and year out. I've never seen another mower that throws grass like that, and you must know that those prisoners didn't pamper them. Yours will most likely have an 'F' engine or a Duraforce 6.5HP. A commercial LB with the Duraforce would have to be the ultimate lawnmower, although the 'civilian' 10424 came very close. I would have said 10550 except it has the Toro Personal Pace drive that irritates me. The commercial will not have Personal Pace - if it's propelled, it'll have a 3-speed.

$75? Get it quick. The wheels are worth that much.