Scalping is more of a subjective assessment of a person in a certain situation, rather than an objective simple designation that covers all scenarios. It's more of a mentality by the individual or corporate body, that allows most of us to label someone (or some company) as a scalper. That scalper mentality is when a person (or company) charges more for a product or service because of added demand, often created by them in the first place, rather than charging the same price that covers their own standard profit margin reguadles of how much stock they have.
Retailers, even the big two (Coles and Woolworths) have a scalper mentality when it comes to their competitive behaviour. They will drive out the competition, and then raise their prices in their newly created monopoly. We wouldn't necessarily label them as scalpers, but it is the same principle, and technically they are scalping by taking advantage of their 'market position'. Generally though, stores like Target and Kmart, will have a national pricing structure, that covers their expenses and profit margins (usually about 40% on top of wholesale), and you can usually find the same price in any of their stores, no matter what their market position is.
The same applies to individuals. How an individual sells a particular item or toy, is how they will be perceived by others. If they have in their hands a rare item or their 'market position' is that they have a monopoly on that item, selling it for more than a reasonable profit margin would make them act like a scalper. It is difficult to draw a line on what exactly makes someone a scalper or not a scalper, but the way they operate and are perceived by the marketplace, is what labels them as a scalper. Some people might not seem like a scalper to everyone, but in a lot of cases it is easy to label a toy dealer as having a scalper mentality.
An example I noticed yesterday while browsing Robotkingdom - they sold out of their first batch of Animated Leader Cases (one Megatron, one Bulkhead), which they priced at US$105. The huge demand, which they even note on the page itself, has enticed them to raise the price on the next batch to US$114 (their wholesale price wouldn't have changed). They would have been making a significant amount of money off the original price, but now added an extra chunk of profit to each sale. That's scalper mentality. They don't need to be selling them at US$114, but because they are one of the first dealers with those toys, they are taking advantage of the demand while they can.
It might be argued that it is just the balance of supply and demand, but if the same happened at Target or Kmart or any department store chain that sells TFs, they'd never raise their prices on the next batch.
That's the difference between retailers and scalpers - the price doesn't fluxuate with demand at retail stores. Us consumers/collectors can rely on prices at retailers, but not with scalpers.
I guess one way of classifying a scalper is reliability of their pricing. If it is by auction or has unpredictable pricing due to demand, they are more likely to be a scalper.