Depends on what it is.
Most parts are simple bolt on's and if there is an issues they can be returned. I did have some Billet Bow Ties for my HHR that did not fit and I worked with the MFG to have them corrected and they sent me the new parts out to check the fit and replace my wrong ones at no charge.
On my Fiero I have a lot of parts that were limited production items from the 80's. They are nice because you don't see many of them if any today. The real issues was that most were not of great quailty and took a lot of extra work to make them fit and look proper,
The fact is there was a time that most parts were made at home. Cams, Intakes, headers, etc. Today most people buy something and just bolt it on. This has lead to many people being able to buy parts they could never make. It too has lead some to do things to cars or create issue that get them in over their heads.
I tend to do what I can as a bolt on but because of the nature of my cars over the years I have had to so my fair share of fab work too. I don't mind if I have the proper tools to do it. But how many people have milling machines etc at their house?