Sometimes they reject your advertiser account because your site looks suspicious (if it's new site and has no or little history) and they are afraid that you would use the traffic to spread malware.
Some companies even require that you get recommendation from other ad companies.
Anyhow... I've tried many companies and until now the ones that I like the most are
FPCTraffic,
Popcash and
AvaTraffic (ava traffic = skimmed).
With $1 - $1.5/k you can get around 10k US/day. FPC is a little more expensive but the traffic quality is even better.