I have had this experience as a buyer: I paid for a set of wheels, but Paypal held the money until I left good feedback for the seller, confirming that I was happy with the wheels. That triggered release of the funds to the seller. If, as buyer, I hadn't left any feedback or complained that I hadn't received the wheels then the funds would have been released anyway after 21 days. Paypal did this to ptotect me because the seller had bad feedback.
If this is the situation you find yourself in, Peter, then it is being done to protect your buyer - as others have said, that may be because the sale price was high, or because you have bad feedback. If you want to wait 21 days before sending, I would suggest agreeing this with the buyer first. It's a big ask, from his point of view. I don't see that it is any more risky to send the frame now than it would be in a normal transaction: it is Paypal who hold the money and they don't have any reason for withholding it unless you or your buyer give them one.
We all had zero feedback once - it isn't a sin. I have negative feedback from someone because the frame he bought from me had a ding, even though the ding was shown in one of the pictures in the listing. It pains me to think that others are looking at my less than perfect feedback score and thinking I might not be trustworthy, but I'm stuck with it. Remember also that your buyer may have joined Ebay specifically to buy your frame.
Hope the deal goes through OK!
Giles O'Bryen
London, UK