Affiliate agreement with niche shops

Hey guys!!
I couldn't find any info about this even though I'm sure is a newbie question.
I have a niche website which is already getting traffic but the targeted country doesn't have related affiliate programs.
In order to monetize it, I was thinking about pitching a few eShops in the niche which are already selling the products asking them to pay me a commission per sale.
I could clearly track clicks but, how to track sales generated by the traffic I'm sending without going trough any official Affiliate network? Any idea?

I'm looking forward for your feedback!

Cheers!