Its simple really, just make sure to be on auction house IMMEDIATELY after some downtime.
Bidding wars usually start when nearing the expiration, so the downtime creates a situation where items are still at their initial starting bid when about to expire.
Search for stuff that normally sells at very high prices, such as:
High dps weapons
+Magic find (rings & amulets sell very well)
Legendaries & set gear
Set a buyout price to limit the results somewhat and start searching like crazy for anything that has a cheap starting bid and <1min-1min left before expiring.
Getting on the second the service is available gives you a huge advantage and you'll easily get tons of stuff you'll be able to sell with a 50% - 100% profit
So far I made an easy 150k like this after EU AH downtime & haven't even sold everything yet