Why must I use my paypal account rather than a credit card?

I have bought several items on EBay recently and used a credit card through the PayPal system. One item I am bidding on now has a seller who will only accept paypal so I had to link my paypal account to Ebay before I was allowed to bid.Having won another item today, I tried to pay with a credit card as before but now only have the option to pay through my paypal bank account. I use my paypal account like a savings account. Any money I receive from sales sits in there for a rainy day so why am I now being forced to pay for my purchases with this account?

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