I'm not a practicing Buddhist but one of the principles of the Noble Eightfold path is the idea of 'right livelihood'. Which essentially means that you should a

I'm not a practicing Buddhist but one of the principles of the Noble Eightfold path is the idea of 'right livelihood'. Which essentially means that you should avoid making a living doing something which does harm to other living things. One of the listed livelihoods to avoid is being in the business of making weapons. This really struck home when I was working for a company doing a lot of DARPA work. I had previously worked in Universities and in a small community organization. I have to say going home at the end of the day at the previous positions made me feel good about myself in a way that cannot be replaced with increased income. It was disheartening to know that if I worked my hardest at the DARPA company, all I did was essentially help the US military kill people better. I eventually left that position to go work for a state university. So I do strongly think that no matter how interesting the problems, and no matter how good the pay one should avoid working in areas that do cause harm to others. Unfortunately the world we live in is a bit more complicated that than one the Buddha lived in, so answering the question "Does what I do cause harm to others?" is a bit trickier to answer. I personally don't know if selling ads meets that qualification, I certainly don't think that merely making a profit does, and I don't know enough about the field to know about HFT and the like. But I do think the important thing to do is ask yourself that question, and if you aren't happy with your answer then it may be time to look elsewhere.