I can tell you from experience....that buying previously registered domains....ESPECIALLY at auctions can be VERY expensive and they usually go WAY overpriced. If you really want to start out in the parking biz I suggest you start with registering traffic domains. They ARE harder to find these days, but not impossible.
Ditto this. You can NOT do anything to increase the traffic to your parked domains or it will most definitely violate the TOS. Best thing is to register or buy domains that have inherent traffic. I am sure you can probably make more money if you want to take the time to develop the domains into websites, SEO, etc. I, on the other hand, run my parking business, am a professional music producer/artist, stay at home daddy to 2 toddlers with a wife in school full-time, and put in about 25 hours a week helping the media department at my church....so my time is EXTREMELY limited. So at this time it makes more sense to me to just sit back on the domains and let the money roll in. I may change my tune a bit in the future, but right now, it just seems like it would take WAY too much of my time to develop revenue generating websites(as I know nothing about developing or SEO) and I don't want to lose the parking income while I am trying to boost income from a developed website. I think you just need to weigh all options when doing these things, and for ME...parking seems to be the best solution right now.