Horton is right, however this is a very tough industry, let me give my take:
1. The are two types of genuine vehicle parts, grey market and authorized resellers. For example Toyota EA may sell genuine toyota parts so expensively that sourcing the same for yourself from an authorised dealer in say dubai and reselling them here makes sense. There are a few shops that sell such, but you have to be careful and know your way around.
2. Then there are those manufacturers who supply vehicle parts to vehicle manufactures, for example denso, aisin, daikin, supply parts to toyota and mitsubishi with a vehicle logo and sell the same without logo and packaging in the market, this are just as good as genuine parts and cost less. But you have to be careful there are many fakes.
3. There are then high grade substitutes, these are parts that are manufactured in areas with strict testing of standards such as japan, this is what you will commonly hear being referred to as "original japan" in spares shops. 555 manufactures very good suspension parts (ball joints) etc, but most of the 555 you will see in shops are fakes from Taiwan etc, so you have to know how to determine the fakes from the originals.
4. High performance parts, this are usually even better than the genuine parts and are made by specialized manufacturers and often cost more than the genuine part, a good example is robbs shocks.
5. There are then low quality fakes of all types and packaging which only the expert(after years of experience) can determine.
6. Then there are parts where you have no choice, such as leaf springs locally made.
Depending on your particular problem maybe i can direct you on how to get the best for your situation.
Ras Kienyeji Man