According to The Cambridge Grammar of the English Language (p1422) the relative lexeme who cannot be used in what it calls pseudo-clefts (also known as wh-clefts). So, the CGEL would claim that neither of your alternative sentences is acceptable.
This is the relevant extract from the CGEL's section on pseudo-clefts:
Range of relative lexemes
Not all relative lexemes are permitted in fused relatives, and hence
in pseudo-clefts. Most importantly, who is excluded:
[30] *Who achieved the best result was Angela.
Instead we need an it-cleft (It was Angela who achieved the best
result) or some other specifying clause (such as The one who
achieved the best result was Angela.)
On this basis, your sentence could be rewritten as
It was those people to whom I've sold your cats.
This avoids the concord problem in your question.
Note that the star in *Who achieved the best result was Angela indicates ungrammaticality in the CGEL.