How much should I charge for an unlocked white 16GB Verizon iPhone 4s?

Jailbroken and unlocked Verizon iPhone. Also- Is it possible to use it with an AT&T SIM Card?

Depends on the condition. If it's like new, you can maybe get $200-250 if it isn't you probably won't get more than $150. Jailbroken really doesn't do anything for most people, so you might want to undo that before sale because someone who doesn't know what they're doing could easily ruin the phone. And no, you can't use it on AT&T. I have heard about using them on Metro PCS however…

You should research the local price. A good place to go is Craigslist and see what they are going for in your area. Keep in mind that the purchaser might not value the fact that it is unlocked, thus lowering the price that you ask for.