Everyone who wishes to buy/build any type of govt controlled/regulated item, should first and foremost be aware of what the law is regarding such item. If you want to buy a parts kit for an upper that, if added to a lower would make it an illegal item, there are plenty of ways to find that out. Websites, and forums like calguns as an example. If you want to buy an entire rifle (SBR) the shop selling it needs to conform to the law, not ensure that you do (other than determination that you are not a prohibited person).
Yes it would be nice if they would inform people that in certain situations assembling some kits into an complete rifle, they could be breaking the law, but AFAIK there is no legal requirement to do so.
If push comes to shove, the DA will tell you that "ignorance of the law is not an excuse to break it."