Hi:
I mean no disrespect, but this sounds like a really bad idea, even if -- strictly speaking -- it may be legal.
An unsophisticated or unsuspecting buyer might be tempted by what I presume will be a "good price" (since the system lacks an OS).
S/he might not know that LEGAL Win7 licenses are extremely scarce now.
So s/he might (unknowingly) become the victim of a scam, purchasing what turns out to be an illegal license from a questionable source (or the wrong type of license), only to run into validation issues later.
Such a buyer would likely lack the skills to sort out the nuances of purchasing a valid Windows 7 license.
Even if you were to "cover your behind" with some sort of disclaimer with the sale, the buyer would end up in a bind as a result of their good-faith system purchase from you.
While a knowledgeable buyer would know what they were getting into, a less savvy buyer would feel cheated of their money (twice -- for the box and for the license) by a deal that may have seemed "too good to be true".
Just my two cents and worth less,
MM