Sold a car and now they want to sue me?

So I recently sold my Honda for $1400. I m no mechanic and told the buyer that several times. I m 19 years old with no history in working on cars. Well anyways the buyer drove the car (I didn t even ride with them, and did not tell them they had a limit on how far to drive.). They had there neighbor come over who "knew all about cars" and he said it was worth buying. Now they took the car to the mechanic a couple days after buying and he found a lot of rust underneath and the brakes were acting up. The mechanic told them that there was no way I didn t know about the mechanical problems with the car. So then she texts me and tells me she wants a full refund. I told her the car was sold as is and that I didn t know about any of the problems. I drove the car 90 miles a day without ever having issues. So do you think she has a case at all? I never told her she couldn t take the car to the mechanic first, and did not hide the car at all. I wouldn t have cared one bit if the guy who "knew all about cars" crawled under and inspected it further. I didn t get a bill of sale but i am under the impression that all sales are "as is" without a written warranty. Also I am in Missouri if that helps.

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