I always put it at the bottom so if a potential buyer wants to add more to combine postage or the main item is not quite what they want they can look at my other offerings.
If it is at the top you distract the buyer from looking at the details of the one they found by a search, if they end up being diverted through all your listings by continually being distracted they may end up forgetting or not have time to actually bid
( Hi Shadeaux )