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In general:

Used car sellers almost always want something like a cashier's check (the kind you can only get from a bank teller, in person, and which costs a few bucks) or cash, meaning something that can't possibly bounce.

(You should have explained that form the beginning!)

Oh, in that case, maybe Paypal or one of the equivalent services? There might be a more straightforward way to use a miles/points earning credit card with an online money transfer than with a physical check.