Category drop down issue

Hi

On my case I will have 2 main categories: buy and sell

On each one I will have 30 or 40 categories.

On the submitnew deal page you can imagine it can be overwhelming to choose one category from one dropdown menu that displays about 80 or more categories….

I need a strategy for divide that two systems (buy and sell) with the usage of memberships or other theme options or I will have to modify the dropdown menu to display only the top 2 main categories (buy and sell) and then after selectiong one present another dropdown with the child categories.

Is there any strategy with memberships, packages or other theme option that allows me to accomplish this?

chhers