John:
Being a rehabber/flipper, I would not pay that much. However, if you are going to keep the grill for yourself, it might be a good find. It looks to be in pretty good condition from the photos and shouldn't require any in depth rehabbing. I also like the wire rack bottom shelf, versus the cabinet style. Some things to look for are excessive rust on the frame overall, and in particular on the left cross member that the cook box attaches too. Look closely at the cook box for bowing out along the front or sides. The top edge of the front should be straight or very close too it. Check the bottom wire rack for excessive rust. Check the larger black drip pan under the grill for holes and excessive rust and corrosion around the edges. Check the inside of the lid for the liner that often times is corroding badly and ready to fall out. If so, you can just rip out the liner but then you risk the lid discoloring due to heat. Replacing the lid with a regular ceramic coated one is usually a better option. Those are some of the trouble areas that I can think of.
But, the bottom line is that paying $150 is high for a rehab candidate. If you are doing it for yourself, that is fine, but it would be too big of a cost to make it a viable rehab/flip candidate. Also, if you are keeping it, I would suggest you just clean it up on the inside well and be done with it. No need to clean it down to bare aluminum. Half a dozen cooks in and you won't know the difference.
Lastly, it has been listed for a week now. I would shoot him a lower offer to start with. He likely already has a replacement grill and wants that one off his deck.