When I started I didn't have any control over / plans for gpucoin.net so that is basically a standalone site.
gpucoinstore.com/gpucoinmarket.com are the same site
gpucoinescrow.com : I wanted to be a standalone site (anyone can use it) but its build on the backend of gpucoinstore
gpucoinapi.com : This was setup on a standalone server for security reasons.

Nothing to do with seo...I think splitting traffic among multiple domains is actually worse for rankings.