The Police does not organize auctions. Bikes that aren't collected by the owners within 90 days from the "Velosammelstelle" are given to the "Ganthaus" (Auctionhouse)...if there are enough.
Next Gant (Auction) is on March 18, 2015.
Information can be found here: http://www.bka.bs.ch/versteigerung-v...tion3_section1
You can have a look at the things offered on the day of the auction from 9 to 9.55 am. They sell movables, vehicles and receivables. Check the site regularly as they will update what will be auctioned.