It is the O2 senser not filter garage said it is misfiiering at start up but drives ok. Could it just be the sensor ?

I bought a car fro a garage last Dec. The engine management light came on after a couple of days. I contacted the dealer who sent me the garage he uses & said he would check it & fund the cost of anywork needed. The car was tested by the garage who said there was nothing wrong with it switched the light off & said it was probably because the car had been stood for sometime. The management light came on again & I have discoverd by another garage it is a problem with the o2 filter. I am trying to sell the car due to financial reasons but no local garage will have anything to do with it telling me that they know where I bought it & it was not faulty when purchased from the dealer. I am concerned that I have been sold a dodgy car who no one will buy or tell me what is wrong with it. Also a friend noticed the make badge on the bonnet is upsidedown. Which in itself is strange. Can anyone offer any advice please.