That's a pretty nice mower with the steel deck and the ability to side discharge, mulch or bag. Honda doesn't offer that anymore. I think that's what the T in HRT stands for. If you buy it, be sure to ask for the side discharge chute, mulch plug and bag, if he still has them. The plug and side discharge aren't too bad to replace, but the bag is pretty pricey.
Kind of hard to tell from the CL pictures, but it looks like the tread is pretty well worn from the back wheels. It has a three speed transmission that may, or may not be bad.
If you go look at it, push is forward on concrete if possible, without the engine running, and you should hear "clicking" sound from the rear wheels. That should indicate that the keys in the drive pinions are free. Pull it backwards and the wheels should spin freely. Then activate the bail to put it in drive mode and pull it backwards. The wheels should stop turning as you pull it back and you should be able to drag the wheels in the locked position. That indicates the transmission is going into gear and nothing is slipping. Since it's a three speed, try that at all three speed settings. While changing gears, check how smoothly it changes gears, and feel for the stop in each of the three settings. You might also hear a light click at the transmission as the shift lever hits each of the stops.
Turn the mower over, with the carb on the uphill side, and look at the condition of the belt. You should be able to see it where it wraps around the pulley at the engine. It should be tight, and it could be the belt is worn and glazed and needs to be replaced, which could also account for the slipping when in drive. While under there, check the condition of the deck, especially around the discharge chute area. That's where it's most likely to rust out. This is a worst case scenario, but you get the idea.
Look in the area above the transmission. I came across one that had so much grass and debris accumulated on top of the transmission, and the clutch cable had come loose where it attaches to the transmission so it didn't work. Again, worst case scenario, but just to show you..
