I think you will burn through $400 very fast with PPC advertising. Are you trying to market to the customers whose cell phones you want to buy? If so, you might consider mobile advertising. You can advertise right on your customers cell phones. What better way to get laser targeted marketing then hitting them right at the source? The costs of mobile advertising are set-up like PPC. You bid on the amount you're willing to spend per click. They will show as many impressions as it takes till you hit the number of clicks you pay for. The cell phone ads show up in cell phone apps.
If you want to buy some awesome training about how to market on cell phones, google "Cell Phone Cash" sold by Maverick Money Makers. I have it and it is very good training. All the videos combined are about 8 hours long. They show you everything you need to know to do it right. The only downside is that their training is a 6 week program, and they will only release the videos to you on a weekly basis. First week, they give you week one modules, and so on. When you are ready to actually advertise, go to google admob and set-up an account, it's free. Then set-up your campaign(s), and you should be good to go. Good luck.
P.S.- There is a great website doing exactly what you are doing called Gazelle. Check it out.
