I tried something like this last year.
I wanted to do a boot sale anyway so took along my business cards and leaflets.
I put a prominent note next to it saying something about although I was happy to haggle with car boot stuff, the window cleaning is my main livelihood and I charge professional rates for it with no haggling (shorter than that but you get my drift). Anyway it worked. I got zero enquiries

.
I just wanted to avoid the hagglers.