I have not looked on eBay, and I don’t know if you are serious or joking. However, I would be highly suspect of any person selling refrigerant on eBay. First, it is illegal unless you hold a valid EPA card. Second, you are not allowed to even hook up gauges without the certifications. Third, there are a lot off jack arses that are trying to pass off even propane as R12 or other high dollar refrigerants.
When recovering refrigerant, they will not be able to recover 100%. The machine and hoses will retain some and this will be lost. It falls under “minimus discharge” with the EPA. When recharging, you cannot get all of it back in due to the pressure in the tank versus the operating suction pressure of your system.
Be prepared to have to buy some. Don’t know what the rules are as to converting to an alternate and changing o-rings and compressor oil. You will lose some cooling capacity, but it may still be acceptable.
Just throwing my two cents in, and you get what you’ve paid for….
Bob