Sometimes if you are accepting multiple items in a single trade and not all items that don't appear on the computer screen, you have to scroll down to view all of the items on that side of the offer. Once you do the "Accept" trade button is no longer gray and you can accept the trade.
I had the same problem and the scrolling issue solved it. Kind of like those websites where you have to read and acknowledge their terms and conditions but can't accept unless you scroll all the way down to the bottom (as if anyone reads those things).