What difference it makes from SEO point of view to optimise sub-domain compared to domain. I mean that domain is not very relevant to sub-domain, like domain name and content is relevant to general description of marketing strategies, tools and services for affiliates and sub-domains are promoting merchants with products from different categories, each sub-domain for different merchant and different product category. Is it better to register separate domain for each product category or it does not make big difference and depends on your work, done to optimise your sub-domain for on-page and off-page factors.
in my opninion, you should use subdomain because - 1. each of the sub domain is promoting the merchants with products which is related to your main domain. 2. you will have some PR advantage from your Main domain.
So, it means that if you own relatively old domain, links from this domain to your new sub-domain should be considered as a noticeable difference and would allow your sub domain to get more weight compared to new domain? But what about relatively long time, after 1 or 2 years.