Well, benchies and proof of your systems built, pictures, etc. You should be able to build them if you are going to sell them, that should be part of the deposit of starting a business. The more you can visually show to verbally explain and support, the better.
It takes money to be successful to an extent, if you can't make a deposit and have financial backing you may want to go smaller time Ben. But if you can afford a few grand at least in your currency to build systems you want to sell, and show, even the 50" Plasma TV kit, you should be able to show it all setup and working. That's part of advertisement. The one part where you're lacking the most ATM is visual/advertisement. I'll give you props for continuing to try, which is good, but I don't believe you have the funds to back yourself.
How will the warranty work? Will the send the part to you so you can then send it to the company for RMA? Will you have stock to replace their part and RMA the part they return? Is there any warranty? What kind of legal documentation do you have in place to cover your ass in sales as to not get sued? If you cannot easily answer these questions with confidence and meaning that eveyone here can understand, you may want to slow down and go smaller. I'm not trying to offend or upset you, but point out the harsh reality of the business envirionment, as it's quick, cut-throat and expensive. You don't want Alienware and HP to notice you unless you can spend some big bugs to compete and protect yourself.
Take a step back, and look at how far you're going, because I believe it's too far for you to handle, whether you accept it or not. If this was local only for family/friends, you'd be okay, but you're talking multi-national in sales and advertisement, and that will kick you square in the ass if you're not on top of what's going on pretty much all of the time.
Please take what I say into consideration, go smaller, do just towers, mid-end, something you can afford to back if your deal/order fails, or you get into a legal battle for refund or something. It can happen, and does more often than not in this day and age!
Keep trying, you're doing a lot better than you were a couple months ago, but still pushing further than your wallet can account for. It helps to have imagination, but use it to make the best of what you can backup. If you're a rich kid, then go for the gusto and do the best you can. I wish you the best of luck and hope you'll take my words seriously, even if only as long as you've read this post.
Think about it.