As far a certified parts are concerned there is a FAR called FAR21.303(b). Which stated the owner can produce or supply his own parts for his own aircraft.( GO read the FAR as I am sure it's paraphrased here). If an owner brought me a set of bearings and they were timkin with the correct #s and he said they were good bearings (owner purchases part and does inspection, performs the Quality Control function) I can install them and sign them off (at my discression if they are good parts) as "installed owner supplied parts P/N ...... in ..... IAW the limitations of FAR 21.303(b), unit ops. checks good" or similar statement and be completely legal in the eyes of the FAA ADMINISTRATOR. The key for this FAR to work is the owner has to be involved- owner buys part, inspects part, buys material, installs part fabricated, fabricates part, assists in some way. Otherwise how could an A&P fabricate a new piece of engine baffling or a hose legaly? He does not have a PMA for the hose even though he assembled it from approved parts. When he assembled the hose he manufactured it if he sold it to an owner for a certified aircraft. ( I see a big FAA spanking on the way!!!) Now if the A&P only replaced one fitting and the hose, that's a repair!!!! So our buearacrats have decided for us (because they know best) that a repaired hose with one old fitting is better than a new hose with 2 new fittings without paperwork cause w/o paperwork the part is "bogus" and we all know bogus parts are bad!!!.. As usual the blame will always fall back of the mechanic who has a licence the feds can go after. The bottom line is the mechanic is on the front line for the aviation safety programs and FAR's to work as the feds can't come out and repair the aviation fleet themselves and must rely on the trust of the mechanic to do things properly and follow the FAR's.
Now that being said, it does not mean the owner can have me install a Holly 4 barrel carburetor on his IO-520 continental, that would be an alteration and is not within the scope of FAR 21.303.
This is why I love the freedom of experimental aircraft. We can do what we know will work and be safe without having to deal with all of the FAR's that are written for all certified aircraft especially aircarrier types.