Aircraft manufacturers publish parts manuals for the airplanes they
build. The law says that you must use the parts specified in the
manual when replacing things. If the part has an industry-standard
number, like an AN bolt or fitting or wheel bearing, you can buy it
from any aircraft parts supplier.
If the number is a proprietary number (invented by and belonging to
the airplane manufacturer), such as a throttle control cable, the
requirement to use only that part means that you have to buy it from
the airplane dealer. The manufacturers are inclined to take advantage
of this and we see some ridiculous prices.
The PMA (Parts Manufacturer Approval) provision relieves us of
some of the robbery. McFarlane Aviation, for example, manufactures
engine control cables, seat parts, hinges, and a lot of other common
stuff that fits common airplanes, and their prices are much more
reasonable. Their numbers are the original proprietary number with an
identifying prefix added to it to avoid the copyright mess on the
original part numbers, while still qualifying as meeting the parts
manual requirements. Their competition often forces airframe
manufacturers to lower their prices, and some of the stuff they build
is actually better or stronger, such as the McFarlane Cessna seat
rails and roller washers.
It's still wise to shop around. Sometimes the dealer's OEM prices
are better than the PMAd stuff.

Dan