For what it is worth, my primary job is a Business Broker... (the car wash gig has been on the side for the last 17 years).
I have sold lots of car washes. Many times the value was based on higher and better use of the land. Other times it was a multiple of cash flow/sales. Figure in the 3X to 4X times gross revenue on the books/tax returns... and that is still a stretch to make the numbers work for both sides of the deal.
A lot of variables, clean books and records, competition, Express washes in the area, etc. Unless you are going to owner finance the deal, the banks (SBA) are the great equalizers in the deals. If being sold as a car wash, the numbers have to make sense.
I would suggest you package and market the deal properly.... you do not want to waste your time with non-qualified tire kickers... You will want to create a sense of urgency and uniqueness to the deal, and to manage the process.
If the broker you hire is worth their salt, (representing your interest and not that of the buyer) they will more than offset their commissions/performance fees...
sjb