I'd forego a percentage of sales and go for like 20-25% of gross profit off of internet sales -- in addition to any negotiated equity. Here's my reasoning: During the early days, you won't make much, but then again they aren't going to be rolling in the cash either. If you take a % of profit, you still allow them to cover the Cost of Goods Sold (COGS) i.e. their 75% of the take will let them buy new inventory. It also lowers their risk -- if you mis-price something or sell at a loss, have large returns, etc. they aren't paying you. Pretty standard commission arrangement there, I feel.
This could give you some security at a later date, too. Going this route puts you on the level of a salesman -- even if they sell the company, new ownership would not likely shut-down a developed income stream (though people commit acts of stupidity all the time). If you work the agreement out correctly, you could be listed as a wholly owned subsidiary, which could be your equity stake. That way, in the event the "parent" company is sold, you would be able to negotiate the sale of the website indepedently and could not be left out in the cold -- if they want the website and its sales, they have to buy YOU out, too, or you could choose to stay on. My terminology may be wrong, but I am sure this is feasible set-up.