In general, damaged pieces are not worth as much as items in pristine condition. But you may be able to fix the desk up to improve its appearance.
You may find a replacement knob on eBay. You'd be surprised at the interesting things you can find on there. Of course, some dealer will ask ridiculous prices for replacement hardware so search around.
A product called Howard's Restore A Finish may help you restore your desk. You don't need to sand so you won't have a big mess. Use the appropriate color to suit your needs. Use 0000 steel wool. This product does a nice job and if the scratches are not deep may even take care of that problem as well as improving the look of your desk.
Follow the directions on the back of the product for the best outcome.