Here's am answer to a very similar question in Quora, a list of things that a non-technical co-founder should be focus on while the site still being developed: http://b.qr.ae/eBYCk2
If after a 8-10 hours programming day you still can do any of these things (and do them well enough), you might not need a marketing&sales&business partner ;)
If after a 8-10 hours programming day you still can do any of these things (and do them well enough), you might not need a marketing&sales&business partner ;)