I do my "Fire Sales" because they catch people's attention, and that lets us make a connection. I'm also running sales with purpose, such as checking whether or not items are priced correctly and getting rid of old stock that I no longer want at my table. None of my sales are anything I can't afford, and anyone taking advantage of them is still paying me enough that I make a profit. I also get a lot of return customers because of those signs, as people wandering the con remember the Fire Sale guy.
What I don't do with the sales is beg. It's not a desperation move, it's a gimmick for drawing customers in. When they're here, I don't push, and they buy or don't buy based on whether or not they're interested in the product (and very often buy something that isn't part of the sale). If you run a sale and seem like you're guilting people into buying something, then you might make some money off of that, but not because they like your work or had a good interaction with you. Cash is important when you're working as a vendor, but self-respect is, too.
Rock on, guys, and never stop drawing.