With the MFG of R-22 being limited in production less each year until it is discontuined I have noticed the cost of R-22 has shot out the roof. Thus it brings out the less than honest contractors and home sellers.
I have encountered A/C units lately during my inspections that I suspected had refrigerant leaks that were never repaired and the contractor just kept adding refrigerant every year. This is very hard to determine and most of the time is speculation only as there is no real proof bottle feeding is taking place.
As the owner of bottle fed A/C units it would be a requirement to disclose this in a Real Estate contract, but it would be rare for full disclosure.