You do not need a PayPal account to purchase items on Depop. If the seller sells through PayPal you should be able to check out as a guest if you do not have a PayPal account.
Sometimes PayPal at checkout PayPal may require a user to create a Paypal account. This is something PayPal do if they suspect the transaction to be high risk, and Depop does not have any control over this.
It's usually very easy to get resolved by contacting PayPal.
How can I use guest check out?
If you select 'Pay with credit/debit card' the app will re-direct you to a PayPal page. Here you will have the option to checkout as a guest in PayPal, which does not require you to login.
This is so that PayPal can record the transaction in case a dispute is opened in the future, and keeps you totally safe when buying!