IIRC this waiting period of 21 days (or until positive feedback is left?) is sort of a temporary thing for new users- my guess is it ensure that you aren't just creating ** accounts to scam people and are actually using it properly. When I started I had this limitation but it was lifted after a couple sales/weeks and then I could instantly transfer to my bank account or spend the $ using my ****** debit card.
So in short that money is safe as long as you do your end properly and also make sure to ALWAYS get a tracking number or delivery confirmation number.
Back when I first started seeking stuff on eBay and was skeptical like you, I would print this shipping label, pack the box, but before shipping I would take a picture with the contents in the box an the label in the picture... I know if it came to it that wouldn't actually serve as proof but it made me feel better about it.