Hi there
The store software can be bought as a turnkey solution so most of the work will be tailoring this to your needs.
I have been at the forefront of e-commerce for the last 15 years and have worked with the military, large telcos , and done work on ebay clones , normal business to consumer and b2b websites.
If you are willing to get in touch and let me know a bit more about what you are thinking I can give you advice on payment gateways, hosting and all the other things needed to get going.