What is the agreed upon method of payment? Did they ask for wire transfer/western union? Or like MisterF said will they agree to use Escrow? At a minimum I would like to see a paypal NOT friends and family payment method so you have SOME third party appeal system in place, even though Paypal can be very difficult for digital items and maybe accounts are not even supposed to be sold on Paypal.
Are they an established business? Can you get a telephone number to call them? What happens if you Google the telephone number? Can they show you their personal social media pages and show you can ID to match, like a Facebook account? It's a lot of money to me, so I would ask for everything. If they are honest, I don't think they will give you any problems. If they are shady scammers, they will just act like someone else is going to buy it and you have to act fast or miss the opportunity of a lifetime.
You can also offer like $1500 and pretend you are going to walk away from the deal, if they accept, you know it's more likely a scam. They already dropped their price $200 so they don't have other people looking to buy for that price.
