The most routinely annoying example of Paypal sliminess is their refusal to allow a user to set the default payment source to a credit card.
If you have a linked bank account, it defaults to that, and you have to manually change it for every payment. This is clearly based on the hope that many users will neglect to do so, and so they can debit money with no cost to them from your bank account (while charging the recipient 3.5% or more), rather than paying the credit card transaction fees (some of which go back to the buyers, if they're smart and have cash back or rewards cards).
Followed by their invariable attempts to sell your their horrible credit cards, dire and false warnings about credit card charges unless you use a bank account, false warnings about foreign exchange conversion fees.
Not the most egregious issues I'm sure (I've never sold anything via ebay or paypal), but makes the whole experience unpleasant.