My experience this year when I was shopping for pumps is that it's cheaper by a long shot to buy a used pump/motor off of craigslist/local for sale ads/etc than any other method. I bought 2 3" trash pumps complete with suction and pressure hoses, valves, etc, for less than you can buy just the pump (without the motor) off of NT or HF's site.
It's not rocket science to make sure you're not getting a lemon. Make sure they start and run well from a cold start and the pump has good pressure and volume of water movement, as compared to what it should have new. If the seller wants to start the pump up without water in the pump and says it's not a big deal, move on, because it is.