It isn't illegal to barter in the USA with services, gold or other things. I myself used to trade computer repair work for chiropractic adjustments (unfortunately my spine was way more messed up than my chiropractor's computer, so I ended up writing a number of checks). I've read one story about a person who was imprisoned supposedly for bartering with gold; without getting into the specifics of that person's case it seems he was convicted for fraud, not for barter.

It is true that some places don't take $100 bills because they are perceived to be commonly counterfeited. I've spent (legitimate, of course) $100 bills in many places, and it's never seemed to be a problem for me.