Submitted by zhuXKET, also found by adriro, cducrest and bin2chen.
RubiconMarket._buys tries to cancel dust remaining offers, but it fails for V1 offers. So it blocks RubiconMarket._buys.
In RubiconMarket._buys, it tries to cancel an offer with dust pay token amount after fulfilling. So it sets dustId so it can pass the RubiconMarket.can_cancel modifier.
But when we move into the cancel method, there is another validation. RubiconMarket.cancel will call SimpleMarket.cancel, and SimpleMarket.cancel tries to return dust pay token amount to owner.
In this case, msg.sender is different from _offer.recipient, so SimpleMarket.cancel will treat this as a V2 offer, while it can be a valid V1 offer. So for a V1 offer, it tries to send pay token amount to _offer.owner when it is address(0) for V1 offers. So it will revert and SimpleMarket.cancel will not work, although it passes RubiconMarket.can_cancel modifier. This will block the _buy method.
In SimpleMarket.cancel, we should refund pay token amount to _offer.recipient when _offer.owner is address(0).
daoio (Rubicon) disagreed with severity and confirmed via duplicate issue #1281
HickupHH3 (judge) commented via duplicate issue #1281:
