How do I make an automated program that interacts with the web

I've currently been working on a bunch of random projects that do actually have some personal use for me and are helping me improve my coding a bit. There's something I'm now wanting to do which involves automated pricing but I have no idea where to begin.

I'm pretty fluent in Python, getting there in Java, and decent with HTML. If this program would be much easily coded with another language, let me know and I'll give it a shot. I'll give an example of what I'm wanting to accomplish.

Lets say that I'm selling Apple Headphones and Iphones on EBay at a certain prices and I want to make sure I have the best price out there. I want to be able to scan a certain page and look at the prices to see if anyone undercuts me. If I had my headphones at $10 and my Iphone at $100 and somebody was selling them at $9 and $99 respectively, I want to be automatically edit my price from my account and undercut them by x amount/percentage.

I'm wanting to expand my coding abilities right now and this would actually help me a lot with what I'm doing. So how would I approach a program like this? I saw some people saying that you shouldn't deal with Web browsers directly but I will need to be logged in on a certain website so I'm not too sure what the best way to do this would be.

Thanks in advance for your help.