modified sell.php to correct the following error:

When user inputs were incorrect, the form values is supposed to be retained for the inputs so the user don't have to enter them again. But the category field was getting reset to 'Books' instead of value chosen by user. Hence modified it.

But I could not test it from my machine. Please check and test this code thoroughly from your side. In case you don't have time to test it before demo, then please ignore it. We can take care of it later. I don't want this to affect our otherwise perfectly working website.