Well I would say it is a risky offer. Like say if you are planning on spending close to 2 months in development time that would mean a few thousand dollars in terms of money and time spent. I guess it would be better taking something upfront rather than a share. Like you clearly mentioned, for a web based business, the total outflow in terms of web development fees is high and can be upward of 10%. Add to that hosting costs and we are talking a lot of money.
Also other than the points discussed above, how would you the splitting of money be done even if there is a share? Would it be on gross sale basis or net sale basis. Mind you there is a huge difference in both. If its gross sale basis then say if product sells for $100 then say at 5% its $5. But if its net sale basis it could even be negative money! You usually make gross profits, but based on how business is modeled, it can initially be negative net profits, so you need to think over that too.
Also a non-compete clause needs to be added. Just incase the business really picks up, then the partner could duplicate the site and might start parallel marketing too, which should not happen.
Given all of the above, I would suggest you take upfront development fees and leave it at that. A couple of years back I worked and promoted a site for two full years (close to 6 months went into development) and then included regular site maintenance, adding regular content, helping users add new products, everything and at the end of the day though the site made gross money, the partner said there were no net profits and hence I got nothing out of it! So it was a total waste of time and effort at my end.