Mont,
If you're not sure, don't do it! You'll second quess the sale from now on, if you part with her now, possibly to the point of regret! You are a fair and honest seller, and you worry about not being fair to a prospective buyer who has spent time and effort to inspect your plane. That's admirable, but would be a lot less fair to you to sell when you're not ready.
You've tested the market, and know you can sell your aircraft when the time comes. Keep her until you find a replacement you can be happy with. You may even find a 172 you can trade for.
Once you've found a replacement, give this buyer first dibs, if he's still in the market. In the meantime, you can offer to help him find another plane, be his "buying buddy". You know you'll enjoy shopping with him, you'll get a better feel for the 150 market (and what yours is really worth), and you may find something you want in the process. IMHO!
You may have to come up with alternative financing for the difference you may not have right now, but that can be paid back once your's has sold, less a month or two's interest!
