I have never actualy had to buy a lawn mower or trimmer, I get them during junk day and fix them. Usualy if they don't start after a few pulls the owner simply junks them. I got a nice Echo trimmer a few years back that way.
The last push mower I got looked to be a couple year old Craftsman. It must have just had bad fuel because I drained what little was left in the tank and refilled it, started right up.
Those little engines are simple, as said above first check spark than move on from their.