Just to echo Frank's point.
Not illegal, but where it comes unstuck is when the buyer gets it with the invoice and the companies name plastered all over the packaging. That's not a good way to generate good feeback on a site like eBay.
That said, if you had the ability to go and collect the item from the shop yourself and ship it yourself, then you are in far more control. To be honest, I've been thinking about doing the same with a certain supplier I know who has the coolest stuff, but doesn't sell on eBay and won't dropship either, but they just aren't close enough to me to make it worth me collecting myself.
So maybe not a bad idea if you can control that side of it. Then the only thing you would really need to be concerned about is stock levels as you will never know if the item will be in stock when you go to collect it or not.
One little tip, if you do go ahead and do something like this, pay in cash and ask for a cash discount. Almost everyone will drop at least something of the retail price if you pay in cash. Of course the keyword in that sentence was RETAIL. You have to remember that you are going to be trying to onsell something for a profit that has already been purchased at retail or very close to it.
My tip, try to find like I have, something as unique as possible in the marketplace so your competition is as low as possible.