You might get some temporary improvement by giving the calipers a good wash with soapy water to remove accumulated brake dust,etc that might be clogging sliding parts. Dont use a stiff brush(and especially not a wire one) or a power washer unless you are absolutely sure there are no rubber seals ,as these are easily damaged. Dont use an air hose as this releases dust. Washing down is much safer for the user.
A soft car washing type brush and gentle pressure from a garden hose should be ok.
Doing this from time to time can help delay such problems, but its no substitute for a proper brake overhaul. And it might not work , or worst still might work on one side but not the other,leaving you with uneven braking that wont pass the mot .(It may not pass anyway as it is )
But selling a car knowing it has existing or potential safety issues (,without making this clear to the buyer) is not ethical, and may be illegal, even for a private seller. If you know there is a potential safety issue that you havnt done anything to fix the fact it managed to pass an mot doesnt necessarily absolve you of any responsibilities. The mot is not a 100% guarantee of roadworthyness.
For instance if the brakes are cold when it starts the mot the hubs etc may still be turning freely enough for whatever checks are made on wheel bearing play etc. And when given a short test on the rollers my still retard efficiently and evenly . But when the new owner takes a longer drive they may overheat, causing brake failure or even a fire.
By the way brakes can get hot enough to burn your fingers or boil off water instantly and maybe smell a bit (Eau de ferodo ) even when in good condition and used moderately. It doesnt necessarily mean there is a problem. But a burning rubber smell ,heat and smoke thats obviously excessive for the circumstances and other symptoms of binding brakes are good indicators.
