I want to be sure I price high enough to allow for those who want to do some negotiating
Price it so that all negotiation leads to quantity buys, even if you're practically giving the stuff away. In a flea market, there's a pyschological trigger of sorts going on when one person sees another person loading up an armload of whatevers at a booth. It generates peeks, and of course peeks are what you need in a flea market.
Would it be bad to have a sign saying "some prices are negotiable"?
You'd get better results out of a sign which reads "Punch me in the head! I'm into that!" Seriously. You will enjoy getting repeatedly punched in the head by random passersby more than you will enjoy what happens if you put up a big sign which says, in essence, that you do not know what any of your stuff is worth, but that you welcome suggestions.
I have some baby and kid socks also that I can price $1 for 1 (3 pack), $3 for 4 (3 packs) or something like that.
Wherever possible, try to round it to "the next bill up" eg. $1 for one three-pack, $5 for seven; or $5 for five plus a FREE (whatever, something on which you've got the margin to count it as a profit.)
Maybe with the used clothes I could offer $.10 off the total for each additional piece purchased?? E.g...5 shirts at $1 each will get $.40 ($.10 for each one after the first) off and only pay $4.60.
Flea market traffic isn't like web shopper traffic, where people have three different shops open in browser windows and are figuring out which credit card has the better miles awards on it. It's people with some cash in their pocket, looking for one thing and inclined for impulse buys. Just round it up -- don't be afraid of quantities. "Who needs ten shirts?" Well, this woman, and her sister, and her other sister, and her friend Fran, collectively need ten shirts. This kind of mentality is far more common in a f.m. environment than the obsessive penny counter. Buy a bunch, save a bunch.
Come to think of it "Buy a Bunch - Save a Bunch!" might make a better banner than "Punch Me! I'm Into That!" )
But I'm thinking people will offer less than priced for the clothes and if I bargain and lower the price, I wouldn't want to also give a discount, so maybe it's better to not offer the savings, but just be willing to negotiate on price??
People are going to want to negotiate with you, and trust me when I say that you never want to tell them "no." If you don't like what they're offering, offer them a quantity discount, grouped items, similar but cheaper item, etc. I know that this might sound weird, but make them tell you "no." Offer them one or two options if you don't like their offer, including simply countering with a slightly higher price. But always leave them to be the ones who say no, can't do that, sorry.
I'm all excited about this meet -- can't wait to hear how it turns out for you.