I'd be very careful — plenty of "free" fonts come with a license that doesn't permit you to resell the font. And what's scary is that sometimes the font is distributed without the license so you could get sued if you "just assume".

My advice is to find a few type designers and ask them if they would would license their typeface to be on the safe side. Either you want to offer a flat fee, or a fee plus a cut of each sale.