The plugin I used to use (and am still using for the 'deal of the day', seeing as it needs to be updated daily) is 'Tradetracker store' by Robert Braam.
Could you tell me why you would redirect immediately to the merchant's site? Has it to do with SEO? I think having separate product pages is nice because of low (or less high) bounce rates, but it does give more duplicate content, which I keep thinking about. Is that why you would redirect immediately?