This is how I choose a market. I have been working on PPC for about a year. I mostly test merchants and markets by putting up a quick campaign on Google Adwords linking direct to merchant website with keyword tracking. If I do not get a sale in 100 clicks I drop it. If I get a sale I run with it and tweek it to make it profitable if possible.

If I push it I can do 5 to 10 campaigns a week. Currently 1 in 10 are profitable

I now know exactly which keywords are generating sales and what merchants convert. If the volume is good I will make a site around it.

If you build a 1000 page site about dogs for example you may find that only 4 or 5 pages make sales. If you know the keywords that convert in advance you can build pages around them and concentrate on getting links to them.

Hope this helps.

Noel