Where Do I Stand?

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http://forums.oscommerce.com/viewtopic.php?p=173926#173926
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I currently run a website for a small business friend of mine and i have been selling his goods on ebay for him on a commission only basis, I basically catalogue, photograph run and maintain everything for him and I get paid as a percentage of the ebay sale.
My friend then asked me about doing a similar thing with an online shopping cart so i started looking around and got hooked on osCommerce. 
It got me thinking that this may be a good way to actually run a business based on this model that I will build, run, maintain the total store for nothing for my clients but get paid by means of a commission on the total sales.
I need a way to tie the client in with me to stop him from using me to develop and populate a store for nothing only to find then that he dumps me, takes all my effort and gives it to someone else. I was thinking along the lines of having a contract drawn that gives me some sort of ownership of the shopping cart and in the event of him wishing to dump me I would be able to just switch off his cart, I have no objection to him having the rest of the site. I spoke to a solicitor and all seemed ok until I started looking at GNU licensing issues and am now totally unsure if I can do this under GNU.
Has anyone any thoughts on this? I fully intend to keep all osCommerce logos and copyrights on the site but can I sell him a software license?
Can you just pass on the the virgin source code without all mods and contributions added?
Can you license maybe the data images strored on the mysql database.
I would like to hear anyones views on this matter.