A seller can cancel any bid (all bids by that bidder on that auction are removed) up until the very end of the auction. What s/he cannot do is cancel ALL bids AND end the auction early without a winner in the last 12 hours (scheduled) if the auction has ever had a bid (s/he can end early to sell to the high bidder at the "current bid" amount).
A buyer can only retract (with a valid reason, of which there are very very few) a bid before the last 12 hours or within 1 hour of placing the bid (whichever is later, but no later than the actual end of the auction).
