There's absolutely no law against. The purchaser is free to do re-sell the pack in any way they want. You'll notice that most multipack individual packs do not have a barcode, which is the way manufacturers try to discourage selling the individual packets. However, mobile cafes, vans etc sell their stuff over the counter without the need for a scanner, so it's not an issue.
We sell loads of snacks and crisps at my uni shops, restaurants and cafes, most of which have come from multipacks. The only other relevant thing sometimes is that crisp bags for example and slightly less in weight than the individual packs you buy from boxes.
My local Post Office sells packs of pens and pencils with the "Wilko" brand, which they admit they buy and resell from our local Wilkinsons store at a good mark-up. Again, there's no law against it if people are daft enough not to notice