I never knew about this until I ended up hitting it myself, so here goes.
You can follow the approx' 200 threads of irate users at the Paypal community here:
https://www.paypal-community.com/t5/forums/searchpage/tab/message?q=21+days#message-listor my short version is thus:
If you're under 100 qty feedback, and/or 100% positive, along with other rules, Paypal can halt
incoming payments to you until such time as the buyer leaves positive feedback or you can prove
the item arrived.
Here's PP's page on the matter:
https://www.paypal.com/c%3Cwbr%3Egi-bin/webscr?cmd=xpt/Marketing/securitycenter/sell/SaferPayments-outsideWhile that sounds all well and good, first off their rules don't seem to apply across the board, as powersellers
and the like with 100% positive feedback are getting pending notices as well. It's not isolated.
So what is the big deal about?
Well if you can't get your funds, and you don't want to use (or in many cases don't have the extra $$$ around) your
own money to ship the item, you have to wait the 21 days to get any funds. Several users are finding that even after the item has
arrived, they've had to call Paypal and take it up with Cust' Support and prove the item was received by way of delivery confirmation.
In addition to this, if you choose to ship before payment clears, the buyer can lodge a dispute and Paypal auto-refunds the amount.
So the seller is once again stuck, and someone made off with your package.
From the buyer's perspective, if you leave feedback ahead of time, that doesn't help if you need to leave a claim due to
a problem with the item or otherwise, because you have to leave positive feedback to allow the seller to receive your funds
in some kind of timely manner.
In the meantime, Paypal already took it's fees out of the total and you're hanging in limbo.
I'm not usually one to make a stink about something, but you sure need to be aware of this if you've got money, emotions,
or valuables, riding on a transaction clearing in a timely manner.
So I dunno, discuss, be informed... suggest alternatives to use.