Bear in mind, all of your "tests" are with very low amounts. If the transactions are worth only a few dollars, it would make sense for them to follow through and give you your cash if it means you convince others that there's no scam. So when somebody sends out 3 btc hoping to get paypal because all of these $5 transactions went so well, BOOM, they vanish without a trace.

EDIT: For those of you looking to get paypal, this site may be an answer https://cryptothrift.com/

Money is held in escrow. There are people with 400+ transactions on there that have no negative feedback. I personally have sold about 1 btc in exchange for paypal through sellers at this site.