My top 3 would be LED's, shaker motor and mirror blades. A strong 4th would be Cliffy protectors. However, you're not going to make any money on any of those. The return on investment lies in your game standing out above the others and people are willing to pay a little more for some mods.
Best investment in selling a pin? Take a little time and take as many pictures as you can of all the areas that you can. Put together Photobucket album or whatever website you use and that will save a ton of time for you and any prospective buyers.