Buying from them is very likely not illegal. Using MacOS X on their computers most likely is. At least Apple is convinced that it is, and they will likely ask Psystar for a list of their customers when this is over 😱
Oh? Is that why was Apple so eager to settle out of court earlier?
Because EULA's are weak, that's why. They're highly vulnerable to any reasonable interpretation of copyright law. Licensing agreements (which is the lone leg EULAs stand on) apply only between a copyright holder, and someone they are licensing to make for distribution. Which is to say, it applies between an author and a publisher, not between a publisher and a retailer (or an end-user). This reason is why EULAs are completely bunk, and why the case law on the subject is so mushy. Very rarely a judge (who has likely been paid off) will issue a decision (often only partially) affirming EULAs, but just as often the opposite happens. This is why companies in general go to ridiculous lengths to keep tests of their EULAs out of court.
Strictly speaking, what Psystar is doing is buying boxed copies from Apple, and then reselling them. That's covered under the first sale doctrine, and is an absolutely protected legal right.
Apple
may win, but it will almost certainly be on DMCA grounds, based on Psystar's defeating Apple's "copy protection mechanisms" (quotes because this is a nebulous gray area because the DMCA is very poorly written) to allow installation on non-Apple hardware.
The customers are however, entirely in the clear from any criminal standpoint. Apple
could theoretically choose to pursue them in civil court for damages, but I strongly doubt it. It could easily turn into a PR disaster.