When Paypal launched over 20 years ago, it wasn't easy to set up a merchant account as a small seller. You'd need to set up full business banking, then get a credit/debit card machine. Card fees were higher as well. Bank transfers were also far harder to arrange than they are now. Don't forget, we're talking about 1999:

• No smartphones.
• Online banking existed but was pretty basic.
• To pay money into someone else's bank account meant a visit to a bank in person.
• Transfers were slower than they are now and/or expensive (at least here in the UK, where I can send up to a few thousand pounds without a fee to any other UK bank account, within 2 hours).

So the alternative was cash or cheque (check). Cash is fine if you're meeting up and not worried about robbery. Cheques are slow, especially if you have to post them, and open routes to fraud.

So Paypal targeted small transactions, including in ways similar to Venmo these days (which isn't as popular in the UK as in places where bank transfers are more hassle).

But Ebay made a big difference: They bought PayPal soon after its IPO and owned it for over 10 years, that's how integrated they were, when other options were few and far between. I did buy things by cheque on eBay, and once or twice with cash (even in the post for very small amounts). And if you wanted to sell online in small volumes, perhaps as a hobby project, eBay was the most likely way to do so. This was before Amazon Marketplace or Etsy, and while off-the-shelf web sales packages existed, they were cumbersome and required sysadmin and security skills.

Now eBay supports credit card payments directly, but still has PayPal integration for buyers. There's Stripe for small sellers with their own sites - but many of them support PayPal as well. With no pressing reason to migrate off it, I use PayPal on eBay and to pay for a few other things (like training sessions and sports club events), though the latter seem to be moving more towards Stripe; I'm told it's less hassle and cost for the organiser.