Have you done any research into how profitable such a thing may be? There are already a few really big websites that offer bulk ticket deals to end users, specializing in that. Even in South America, where most of my traveling experience was made, you can get ticket deals online from sites that operate in several countries. Not to mention deal sites (like Groupon) that may offer tickets as well. Seems to me that travel agencies these days need to offer a lot more than just airplane tickets. And even in that segment the competition is fierce. Just look at any Tourism directory on the web, and you'll find hundreds and thousands of small operations. And you'll also see a lot of dead links going to companies that have gone under.
If you are looking at such a specific niche, maybe you can market your venture as something other than strictly a travel agency? I'm sure there are other ways to get cheap tickets, since there are several non-travel agency companies that sell them.