eBay sometimes defaults to PayPal as your payment if you've used it before on eBay. It's sometimes easier to simply pay as a guest and then enter your card information. Sometimes, the eBay site will keep redirecting you to the screen where you pay via PayPal even if you request otherwise. If you're having this issue, try clearing your browser history as well as your cookies. This may clear up the issue. If you prefer not to use your PayPal account for eBay, do not link your eBay and PayPal account to begin with. If your PayPal is linked to your eBay account, this can sometimes result in PayPal being your default payment option. If you've already linked your PayPal and eBay account, try creating a different eBay account with a new email.

TL;DR in Vietnamese:
Thanh toán dưới danh nghĩa khách nếu đã từng sử dụng PayPal trước đó. Thử xóa lịch sử trình duyệt. Không liên kết PayPal và tài khoản eBay.