The PA28-180 is
not on a permit, MJ.
Like it or not, you do have to use a MO to do stuff like changing engines
Regards actual quality of work, yes this varies greatly, which is partly why I do my own 50hr services (it also saves me a huge amount of money). Usually I do them with an A&P/IA friend of mine, not least because some of it is a 2-person job, but I could do them on my own and have done so if I can get help with the cowlings.
Changing an engine is a fair size job. It needs a crane, and practically speaking needs a hangar, and this makes it out of reach of the vast majority of aircraft owners because they are not allowed to do work in their hangar.
Also, engines are often delivered without the accessories attached (or with them packed loose). This is more stuff you have to do correctly.
Anyway, we are picking over a silly advert written by a person with poor grammar, and I very much doubt he or his fellow pilots actually changed the engine themselves