I had plans to do this once. Trouble was I wasn't 100% certain if the 90 days was from (a) the time it was listed, (b) the time the auction ended (c) the time of the winning bid, or (d) if it was a BIN auction, was it when it was bought. So I chickened out and didn't bother. After 90 days I had calmed down anyway!

eBay used to have a link that would allow you to leave feedback after the 90 days were up but this has long since vanished (on .co.uk at least).

I too would be interested in software/website that would do this for me.