1. 2Checkout is a great option with automated interface: http://www.2checkout.com
2. Usually comes with a DS. If not, look into cPanel or Ensim.
3. Look at 2Checkout's forums. Completely automated isn't a good idea (fraud). Look into an order form and MS-Excel if you want simple or build your own solution (best bet for long run).
4. May also want to invest in good spell checker
LOL - thanks for the chuckle RackNine with your advice #4.
Mastwolf - honestly, at that level of intensity, do yourself a favor and outsource the whole thing. Go for a full blown reseller suite rather than try and build your own. In the short term you will certainly save money, and most likely in the long term as well. Not to mention about a million headaches!
There are so many great reseller programs out there today that unless there is something in your setup that is totally against the grain (intentionally) doing it yourself is really just another obstacle.
Just my $.02 and I don't even know if I could get that for it. Good luck.