I've bought and sold clothes at a car boot sale and never saw anyone provide changing facilities. Caveat emptor, I'm afraid. I will be honest about sizing, and say "this says 10 on the label but its a tight fit" or similar.
I'm thinking of starting an ebay/car boot sale business selling clothes. However one thing I'm unsure of is whether I'd need to work out how to make some kind of changing room for the bootsale. The clothes would be new.
Are people happy to buy clothes without trying them on?