A great car search engine definitely helps but it's a very tiny part (may be 10%) of buying a used car (speaking as someone who bought a used car 8 months back).
The most time consuming part of buying a used car is to check the car in person (and it's difficult to schedule appointment with an individual person), getting it inspected by a mechanic (If you don't get it inspected by your own mechanic, you are an idiot. I know I was an idiot once.), negotiating the price, paying the agreed price (it's a BIG pain if the car's loan hasn't been paid off).
I recommend CarMax (http://www.carmax.com) (no affiliation what so ever) for used cars because they actually take away all the pain points which I described above.
[And if you are looking to buy a new car, I would definitely recommend carwoo.com. I helped my friend buy a new Infiniti EX at an unbelievable price.]
In all seriousness though, if you spend 20hrs to save $2k, you're making $100/hr, which is better than most programmers make, even.