AJAX :: Web User Control And Modalpopup Extender?

I am quite new to asp.net. I would like to ask for some advice on how this scenario can be achieved. Here is the problem:

I have a 5 gridviews on my page. In each gridview, I have two important columns. They are SellerID and BuyerID. Because I already have too many gridviews on the page, I want to load a webuser control with a modalPopupExtender when either SellerID or BuyerID is clicked. That is to say if a user select a SellerID, the details information about that SellerID should be loaded with DetailView from the user control and if the buyerID is click, the details information about that buyer should be loaded with another detailview
from the user control as well.

My problem is I do not know how to get the Selected Value from the gridView since the DetailView is in the WebUserControl. One more thing, both SellerID and BuyerID are in the same GridView, so how can we distinguish when a user select a SellerID or vice versa.