You can't.
Most sellers,certainly the smaller ones with the interesting stuff, do not have merchant accounts, which have a heftly monthly charge in addition to per-transaction fees that are similar to Paypal.
If you want to add your credit card to your Paypal account, just go to the PP site, here:
https://personal.paypal.com/ca/cgi-bin/marketingweb?cmd=_render-content&content_ID=marketing_ca%2fmy...
which should allow you to add your credit card to your account.
This gives you double protection: Paypal's Buyer Protection and also your card's "chargeback" programme.
