AC Units not keeping up with temps as unit gets older.

[Forrest J Green]

Thinking about replacing condenser only but honestly don't know the affected part. The system used to cool adequately but now can't get below 78 on a 90 degree day.  I have gotten quotes for replacement but very expensive compared to cost of materials.  I can buy a 2.5 ton unit with furnace on ebay for $1750 but yet installers want $6,000?  so they want $4,000 for labor?  I've gotten an installer to install my parts before and charged me like $1000 or so.  Trying to do that again but hard to find these kind of people.  Any thoughts on all this?  or any thoughts of buying a used condenser etc which I see cheaper to buy me several more years  ?