Is there any website of where I can look up the origins of a Chinese character?
Yes, almost certainly a duplicate question.

But you raise an interesting question: why does 讀 have 賣 (mai4) on its right hand side?

In actual fact, the right hand side is a character pronounced yu4 (which looks similar to 竇 but which I cannot find in my IME) which was simplified to 賣 in the clerical script. Why does yu4 not sound much like du2? Well, according to Baxter's reconstruction of Old Chinese, they did: 讀 (*lôk), 賣 (*luk).