Lately I've been doing a lot with the eBay APIs and more specifically the trading APIs for listing items. (I've also used selling apis a lot in the past when working with the ebay deals placements while at eBay). Some of the quickest results can be acheived with the use of pre-entered products found using isbn, ean or reference numbers. The trick when comparing with existing competitor items and trying to re-find these numbers, is to find the item using individual trading api calls and your token, as the shopping APIs don't often give quite enough detail about things like Ean numbers.
I'll possibly be showing some of my out-of-normal-hours work at this months eBay seller Meetup, happening this thursday. So much is happening in the ecommerce and ebay space at the moment, so it will be good to discuss the month over a beer. If your interested in eBay selling, RSVP and come along, it's every month on the third Thursday of each month -http://Meetup.com/ebayuk