Use a escrow agent on this forum to hold the bitcoin funds from the buyer until you've delivered the good.
They'll charge you a fee. There's a few very good, well know, well respected escrow agents here.
I was looking for a list of escrow providers, there's a thread some where, but I can't find it. If you want a list of providers post a thread in Services as "Seeking Escrow..." OR go to the Collectibles board and see who's offering escrow for the big physical crypto coin transactions that are going on there every week. Any one of those folks are trust worthy.
I do not understand how to make $4000 out of a $6000 coin? Like I said, brand new.
Same as you make a quarter out of a dollar. 1 Bitcoin is divisible to the 100 millionth of a bitcoin (we call that smallest unit a satoshi).
When the buyer agrees to the purchase, they'll send you $4000 worth of bitcoin, whatever the rate at that time determines that amount to be. (i.e., if Bitcoin is worth $6000, the buyer will send you
BTC0.66666666; If Bitcoin is worth $10,000, the buyer will seend you
BTC0.40000000; And so on...)