1. Make a Brush, a solid world brush where you want your buyzone to be.
2. Go To the browse textures function and in the "search" field, or whatever it is, type "nodraw". This will give you the nodraw texture. Cover your brush with this texture completly.
3. Go into one of the 2D views, right click on your brush and then select "tie to entity"
4. Make this brush a "func_buyzone", then adjust the properties for which team.
5. Batta-Boom Batta-Bing, Run It and there you go. Any Questions, feel free.