Quick Answer:
When sellers insist on a PayPal payment that is not funded with a credit card, they basically want you to pay with the money that is in your bank so that way they won't have to upgrade their PayPal account and get charged a fee, which is usually 2.9% + $0.30 per transaction. Personal PayPal accounts can accept up to $500 non credit card payments a month without getting charged a fee.

Details:

There are 3 types of PayPal accounts:

1. Personal - are for use by individuals only and cannot receive credit card payments. Personal Accounts include all of our Core Features.
2. Premier - are for members who have a high transaction volume, need to accept credit card payments, or would like access to our special features. Premier Accounts include all of our Core Features, as well as our Premium Features. Premier Accounts may operate under an individual name.
3. Business - are for business use only. Business Accounts include our entire Core Features, as well as our Premium Features. Business Accounts can operate under a company or group name.

Typically, sellers might say something along the lines like this when they are want to save money on PayPal fees:

• Please do not pay with your credit card through PayPal.
• I cannot accept credit card payments for this auction.
• Bank transfer only.
• No credit card payments allowed.
• and many more different ways basically stating they don't want you to pay with your credit card.

Other Information:

• A seller/buyer is allowed to have one Personal account and one Premeir/Business account.
• This issue about sellers not accepting credit cards hasn't been that big of a problem, and I have been seeing it less and less.