PeakMonsters executes HUNDREDS of bids every day and regularly saves users 4-50% off of the list cost of those cards on the market.
Just a random assortments of bids from today is a good example to prove the point. Every one of these cards were purchased well below the listed price on the market.
So if you're excited to get a couple percent back on your market purchase you should be even more excited to get your cards at 5-30% more below that.