Or is there some custom in the supply chain by which these firms waive that right for priority in sourcing? In which case, this sounds like an unethical buisness practice solely intended to create market inefficiencies...
Or is there something else to it?
There is a difference between unilateral refusal to sell, and selling on condition of non resale. As far as I am aware, the practice of conditional sale like that isn't actually backed by any legal teeth outside the realm of a contract. Once you buy the thing, it is yours. Now the contract could cover the primacy of sourcing, but not the item itself, which would have the same badic effect I suppose.
Believe it or not, "we just won't sell to them" is a surefire way to stir up trouble, because then people start asking pesky questions like "why?", and if the answer given isn't satisfactory, leads to going about and collecting data; turning it into a public interest sort of thing. There is no way that blacklisting sales without a darn good reason is ever a good thing.
Companies are free to negotiate terms (prices, delivery guarantees, priorities, etc.) for selling things how they like, generally. If it's in their best interest to not sell to someone, or to charge them higher rates / offer fewer discounts, that's what they'll do.
You can also do this somewhat risk-free if you either first sell on ebay, before actually buying the computer, or if you sell on ebay within the return period of wherever you bought the computer.
That way if you don't find a buyer at the desired price, you're just back to square one, and not stuck with hardware you don't need.
I bought an art skateboard that had production problems (wont ship out to original buyers TBD) but it's already listed on ebay from scalpers.