We source exclusively online, so it can be done. We do not use deal websites or the major stores like Walmart.com, etc. We get pallets at a time. It still takes time to decide on the good products though, but I admit that it's easier to do from the comfort of home. We started like most everyone else, doing RA in Big Lots and similar stores.
As for UPC's, Amazon TRIES to get one ASIN per product. But many manufacturers will use multiple UPC's on the same product. Software is famous for this. Software manufacturers would put a different UPC on a product going to Best Buy, and one going to Walmart. Even within the Walmart & Sams Club system, they use different UPC's on the same product. Helps keep someone from buying something at Sams and returning it to Walmart. Anyway, in a perfect world, there would be one UPC per product, but that's not the reality and Amazon knows this. Heck, there are even some manufacturer's who use a UPC on a case and another on the product, and some that use the same UPC for the case & the product. You can get items with different UPC's merged into the same ASIN, but it usually depends on the seller central rep. We have done this when we have one item that we sent in on two ASINs. Once one sells out, we try to get them merged so our inventory is automagically moved to the ASIN with the better rank. We have also done this when we had inventory on a dead ASIN and the same product on another ASIN had a better rank but higher price than us. (Yes, I'm the guy who did that to you.)
So don't worry too much about UPC's. As for incurring Amazon's wrath, as long as you don't do anything blatently horrible, you should be fine. Amazon is mostly focused on customer experience. So don't game Amazon or the customer and you'll be OK. If you are creating ASINs using UPC you purchased just to eliminate the competition, you're gonna have a bad time. If you send in product that is not IDENTICAL IN EVERY WAY on an ASIN because you thought they were 'similar enough', you're gonna have a bad time. If you send in counterfeit products (like 'name brand' products you bought out of China), you're gonna have a bad time.
If you don't want competition, create a bundle. Or better yet, private label. There are plenty of legit ways to do this profitably. My best advice is to forego the books and seminars and find a mentor. Or better yet, several. Someone who makes money doing this. Learn from them and then move on to another mentor.
In my experience, it's better to search on Amazon for something by name, not UPC anyway. All the 'apps' use UPC, but I have better results going into seller central, add a product, and searching by name. It will show you what isn't on Amazon currently, but exists in the database, and what order they sell in. So it's much more helpful. Doing this, I have found better ranked ASINs for the same product that was out of stock, so you couldn't find it on Amazon.com. I was able to take over the ASIN and already had a better rank than the existing one. It will also tell you if there are bundles or how they did.
As for profits, we gross 6 figures with about 50% net profit after expenses. So yes, it can be done profitably from home.