Bunch of places really.
In purely local reach there's craigslist and f/b Marketplace. Armslist may have a section for accessories, but you will be spammed.
GB is an option, as it has more national reach, but, you will have to figure out how to box the thing up to ship them, quite probably across the nation. As moderate-weight items but out of 'normal' parcel dimensions, shipping is going to be $30-40, factor that in, or make it clear buyer pays.
Also national would be eBay (you will want to restrict the auction to the US, too, to avoid international issues about 'arms trade').
The classified section here, as suggested above is an excellent idea. The shipping issue will apply, though.
Definitely set them as one sale per each. Write a detailed description, then Print It Out. Put that aside. Read it tomorrow, then rewrite the gibberish out (trust me, there's gibberish there). Then post the sale listing. Yes, this is a lot of work, but sales are hard work.