Weighing the fabric makes total sense to me for a stash-sale.
No need to separate or have different prices for thin cottons or thick; other fabrics such as fleece, minkee, upholstery, corduroy, etc.
So much quicker and easier at the check out!
And would make for a lot less stress on the day-of!
Should be no problem, so long as you are clear on the "rules" ... and comfortable with the price.
Perhaps a good big sign or two at the sale stating the price/rules!
Might even include it in the advertising/promotion you do.
Buyer has the responsibility to measure themselves or guesstimate. Whatever works for them!
Let's them amass a whole bunch of stuff and you don't have to go through it when they go to pay.
You're not a quilt store, so don't even consider cutting.
They like it and want it ALL, or they leave it for another person .... or find a friend themselves to split it with after they leave.
Look for other ways to simplify your pricing .... eg. all patterns are the same price.
I've been to too many sales where it becomes too frustrating as everything is a different price and those helping can't keep it straight what the price is!
Good Luck!