Example from what i just did:
Prices on gems are low, after a weekend of reposting and undercutting. There where 5 types of gems that were under 15g each, with 5-6 listed. It'd be dumb to undercut those. So I bought those out and reset them.
Even if you are not JC thats a good opportunity: sunday night is like the busiest day for gems, so monday morning there can be found quite some bargains on the AH.
Another thing you can pay attention to: scan AH half an hour before the reset, select auctions on duration, and make a bit on stuff that's going to expire in during the reset. Been able to get some sweet deals when i happen to be around.