Back in the day grass operations were part of the training. When I rented the flight school owner would approve grass operations on a pilot to pilot basis.
Returned a rental back at the airport,they didn’t come to pick it up for two days which they tried to charge me for. Also a reservation is not a guarantee of a car being available.
Have done it both ways,prefer to sell on my own,brokerage fees are to high for what they provide. You should have no problem selling ,your plane is usually sought after in the market. Put a feeler out on barnstormers.
If your mechanic isn’t available to do the pre buy find a shop in the local area who hasn’t worked on the airplane. After agreeing on a price,use AOPA affiliate company’s for total search and escrow. I would have my mechanic review the log books.