This close reason should not be added at any point, because it is doubly harmful.
Content-wise, we should not restrict the site to questions that have the form “what X should I buy?”, and we have not so far done so. We also have questions like “what criteria should I consider?”, and this is just fine. A site should be built around an audience or a topic, ideally both. Our topic is computer-related hardware (exactly what that means is a debate for another meta thread or three). We should at the very least include in our scope all “pre-buy” hardware-related questions. A better scope may even be all purely hardware-related questions (as opposed to questions involving software, e.g. driver installation or development), but there we'd go into electronics tinkering which I think is outside of our focus.
Form-wise, this site is not Super User's toilet bowl. Defining a site's scope as “stuff that isn't allowed elsewhere” is harmful, and this is part of the good advice given by Stack Exchange about site scope management.
By the way, whatever definition of hardware we end up with, it's clearly going to be far broader than Super User's restrictive definition (Super User is pretty much restricted to PC hardware and peripherals; for example smartphones are clearly on-topic here and clearly off-topic on SU). So we couldn't just say “if it doesn't have the accepted form then go to SU”.
Let me mention the obvious precedent of Software Recommendations. SR only accepts questions of the form “what software should I use to do X?”. But its scope is not defined as “what's off-topic elsewhere”. There are plenty of Stack Exchange sites that accept software recommendations (for example, Unix & Linux has no qualms with them, we just apply the usual quality control and it works fine). SR ended up with a requisite question format largely because it was an experiment to see whether such questions would work on Stack Exchange. The result of the experiment is that the questions do work, but the site as a whole doesn't work so well — SR has the lowest percentage of answered questions, and the lack of a focus of expertise is definitely part of that. It would be difficult to tweak the scopre of SR to make it attractive to experts, because so much software is of interest only to a small community (Windows users, accountants, etc.). For Hardware Recommendations, there is a rather clear focus: computing hardware, with tags to indicate the type of hardware. SR ideally redirects askers to the site where they'll find the experts. For HR, that site is HR itself.