I'll speak on reselling, not so much on scalping.
Last April we decided we needed a way to make a few extra bucks. I have been on eBay since 1998 and have always sold this or that every now and then so I turned to that to really try to do it more often. As the year went on I started to learn more and more. My method is to search for items that few others have listed, if any at all, and price as high as I can and accept best offers. If an item isn't for sale anywhere, I will create the market value for it.
In the end I did almost $5k in December alone from my eBay reselling side job and did $20k from April when I started up real heavy last year through December. My goal for this year is at least $30k, but I'm really shooting for $40k.
As an example, just today, I sold a hat I bought for $1 for $75 and I sold a t-shirt I bought for $1.50 for $79.95. It has been slow in January, but I think things are starting to pick up now. A few weeks ago I sold something I bought for $2.50 for $240.
There is more than enough "product" out there for folks to do this, and considering most don't want to do it as much as me and others, it doesn't matter if there are many other small time resellers. Our goal is to build our eBay side thing to the point where my wife can quick her day job to take it over for the most part (except for the shopping for new items, I love that part especially). I'd like to get it to the point where we can both quit our day jobs but with state health insurance I'm locked in unless we somehow are able to ramp it up way, way more than we have it at now.
I may start a reselling OT sometime here. I've been meaning to but haven't ever gotten around to it.