I have been both a seller and a buyer on ebaY since 2000.
As to ebaY, it is the sellers there you have to be sure are trustworthy, and that is
NOT all that hard to do: If they are sellers with lots of positive feedback, there is
usually not a problem. But it can happen that products are not as advertised or are not exactly what you expected. Unless you are very anxious about getting a product, either because of the good price, or because you are in a hurry, be careful. Make sure that you have enough information about the item to make an intelligent choice. ebaY has a pretty good system for potential buyers to check up and ask questions of sellers. And there is a good permanent record of every seller's feedback to look at. All that will stay recorded.
Now,
about PayPal. This company is as trustworthy as your bank. But as with everything else in life, it is necessary to learn how best to use that payment service. The reason for their success is partly because when you use PayPal, none of your payment information goes out to anyone else. It is safer than using a credit card in a store.