As Paul said, sounds just like an affiliate set up really. You can set up an Amazon store on your own domain, people can put items into a cart, on your domain, but they pay Amazon.
Just make it clear on the site and in any ts and cs, to simply make your life easier if things did go wrong with the end seller. But I can't see any legal issues.
Thanks Sarah

I thought it should be ok. I'm not doing the full drop-shipper thing, payments go directly to the supplier who will post and package the orders with their own branded stuff. Even the invoices will have their details on it and I'll point all this out in the terms and returns policy stuff.
If it works it'll give a good boost to their sales and a bit of pocket money for me.
They're nice people so I don't have any concerns over using them for orders. In fact I even haggled the commission rate *down* because he was being too generous with it and I wanted to make sure it was set at a realistic level and didn't eat into their own profits too much.