Flippa is the most popular. It's a marketplace for selling websites. What you could do to maximize the sell price, is to run it for a while and demonstrate that it generates money (if it's that kind of a website). There are people looking to buy small but profitable business.
If you google for website marketplace, you'll find a lot more. Flippa is the one that I can recommend. There were some popular websites sold through it.
The is a web applications stack exchange, you might better ask there. That said, there's no one place I know of for such a thing. The key thing to remember when selling technology is the technology is no use without the people. You have to sell the team and the technology. If the team isn't for sale, you can pretty much forget it.
Yes, I also think Flippa is the most popular one.
However, don't expect to make real money from "pure source code". What people look for, especially in Flippa, are websites with real users and, possibly, documented revenue.
If you list your web app there, you'll certainly be asked for these data.