PayPal became unavoidable by solving something the card networks had declined to solve for thirty years, which is how a private individual takes money from a stranger.
Before it existed, accepting a card payment required a merchant account, and a bank would not give one to somebody selling a camera lens from a spare room. The checks, the paperwork, the monthly minimum, and the risk assessment all assumed a business with premises. So millions of people who wanted to sell one thing to one person were left with cheques in envelopes.
The move was to become one enormous merchant and let everybody transact underneath that, which meant taking on all the risk the banks were refusing.
That is the whole company, and it explains the behaviour people complain about. When a payment turns out to be fraudulent, PayPal is the party that eats it. So it monitors accounts, freezes funds on patterns that look wrong, and holds money while it decides. From the outside that reads as arbitrary and occasionally ruinous, and for individual sellers it genuinely is.
From the inside it is the only available response to having promised the card networks that this would not become a fraud channel. The company sits in the position of a bank without being regulated as one, which suits it on the compliance side and leaves customers with no ombudsman when a decision goes against them.
The other development this year deserves less sympathy. The card schemes have introduced a verification step that redirects you mid-purchase to a page asking for a banking password, in a frame, on a domain you have never seen, with no way to check it is genuine.
That trains people to enter credentials on an unfamiliar page when prompted during a transaction, which is precisely the behaviour every phishing attack depends on. A security measure that teaches the exact habit the attackers need is worse than no measure, and it was designed by people who should have known.
I said in the spring that payments move when one party gets enough leverage to stop asking permission. This is the same lesson from the other end. PayPal did not negotiate with the banks. It went around them and became large enough that the negotiation happened afterwards.