Does subdomains increase ranking in terms of sites linking back to the main domain? Please contribute
Danny pretty much covered everything I would say... but I will add - subdomains do little for you rank-wise unless they are independently large repositories of information.
As search engine treat them as separate websites if each is but a few webpages, each is but a small website and small websites almost always have difficulty ranking themselves, let alone help in ranking another site.
I wouldn't recommend do this at all if the motive is to develop "link" for your primary domain, your'll get more benefit from straight directory trees by adding "breadth" & "depth" to your website.
<added>A thought noting unique shop owners... while it may appear to be an advantage to sub-domain individual shops, if you intend to promote each via the main domain, these will "all be treated as outgoing links" and unless each shop owner intends to develop their own linking strategy you will likely lose more then you gain.
On the other hand, if you don't link to them (promote) each sub-domain is "orphaned" thus limited potential of being indexed in search engines let alone remaining there... (as such limited credit for you) but you can always develop their linking strategy for them. Food for thought</added>