I've noticed that there are all kinds of cards, blades, and other parts of ASIC mining rigs for sale on eBay. Is it possible I could buy some cards and blades and build my own high end mining ASIC without having to pay for something brand name or design my own (which we all know isn't going to happen)?.
Technically, sure, you can cobble together parts from bitcoin ASIC manufacturers and make your own farm.
Practically, you are better off going to one of the "something brand name" guys and getting a mining rig. The bits and pieces that you see around on various auction sites are most likely out of date and will cost you more in electricity then you make (not to mention possibly faulty if they are selling them in bits).
Of course, depending on where you are, the chances are that even with the latest gear you won't actually turn a profit.
The mining industry is very cutthroat these days and profit margins are thin. The rig turnover is also very high with miners replacing their rigs every 6-8 months in order to get the most efficient equipment.
I can see where you're coming from; most prebuilt computer systems are awful.
However, most mining units are sold without power supplies, so almost all the time there's some level of assembly required. Pre-built versus "brand name" isn't a useful distinction. I suggest that you make a price comparison based on the hashes per second, power usage, and price.