I built such a site 3-4 years ago with full integration with Eldorado - product feeds, stock check in real time, automatic order submission to them, automated shipping notifications etc etc. But, problem was the site could only be used for that supplier and no other products could manually be added because it required A LOT of extra situations to take into consideration. For example, how to differentiate products from supplier_1 and supplier_2 and only send s_1 products to them. It could be done, but then there's a question of shipping costs and calculating different shipping costs and merging it all together. Then shipment notifications - if s_1 sends it on Friday, and s_2 on Tuesday, we would need to be VERY careful with those notifications. There were some other problems as well and way too many complications to make it worthwhile so the site continued using just one supplier and complete integration.
Any XML feed IS usable, as long as there's properly structured data to have something to work with. The general assumption is it WILL be OK, but I've recently witnessed an XML file with two blocks of data, where it should've been 16... Supplier's response was "split it yourself based on keywords - if it says description:, that's the description field and it ends when you get to the price: keyword". :D I think his 6th grade son built their XML "feed" as part of his school homework...
