I think this sale of 20 btc is a fake, show me this 20btc transaction to prove other wise. If not it is a scam.
You are actually right about fake deal and fake vouch, i mean anyone can make a new account and show that he bought/sold bitcoins from/to his 2nd account and i guess you have right to ask for proof of transaction that happened between buyer/seller, if they fail to provide it, they are probably setting up a scam.
I mean i've been into currency exchange business from many years, i have seen peoples making fake accounts and doing deal with their main account and building fake reputation to do big scam one day.
Ps: pinwheel bro i think there is some misunderstanding between "providing item that you don't have" and "safe trading". Actually it depends on persons reputation.
Scam:If someone with low/no reputation doesn't have something and he's asking for payment first.
Possible scam:If someone doesn't have something and he's taking payment first to provide item.
Safe Trading/Business:
If someone (with good reputation obviously) doesn't have item but he agrees for a deal and ask "A" to send money to escrow, then he buy's item from "B" at low rate and sell's them to "A", takes payment from him and then makes payment to "B".
^ Here if person doesn't have good reputation, he can take payment from A and scam person "B" easily.