I strongly suggest you avoid the cheap SSR's from China that are on ebay. Stick with quality manufacturers like Crydom.
That said, it is perfectly acceptable to use a 240VAC rated SSR on 120VAC.
Another suggestion would be to look for "zero crossing switching". This isn't necessary, but it does make the SSR's job a little easier.
FWIW, I'm an EE that specializes in power conversion. That doesn't mean I don't screw things up on occasion, though (there's a reason I picked "MagicSmoker" for a name ).