Scalping, in the context of toy collecting, refers to the practice of deliberately buying multiples of the same (usually "rare" or "popular", oftentimes both) toy with the intention of creating an artificial scarcity. Subsequently, the scalper will sell the toys again (usually on eBay), for a price much higher than the original store price, knowing that the high demand, coupled with the rarity of the toy (which is at least in part due to the scalping), will result in buyers being desperate enough to pay the prices the scalper asks for. In essence, in other words, scalpers are douchebags.
Origin of the term
The term "scalping" in its original sense refers to the practice of removing the scalp of a human being, usually a defeated enemy, to claim it as a trophy. Later, the term would also be adopted to refer to a practice of market manipulation in the context of trading in securities and commodities, not completely unlike what toy scalpers do. "Scalping" is also sometimes used to describe the practice of ticket reselling, which is very much like toy scalping.
How scalpers work
Being a scalper means having a good advance feeling for which toys will become popular and ship in low enough numbers to make scalping actually a profitable business. Nothing is more annoying for a scalper than buying several dozen Alternators Sunstreaker toys and then finding out that Hasbro just sent another huge shipment of the same toy to stores. Obviously, it's hard to sell a toy for $50 on eBay when the same toy is warming shelves at Wal*Mart stores that offer it for $20.
Many scalpers actually arrange their daily time schedules around their scalping activities. They know when a truck will arrive bringing in new stuff, and then take all the "hot" toys straight out of the case. As a result, many of those toys never even end up on the store shelf (or the peg) to begin with, hence making it outright impossible for normal customers to "compete" with the scalpers (e.g. purchasing the desired toys at the store). Some scalpers are actually store employees themselves, which helps their cause even more.
Scalping in other countries
Scalping toys is not limited to North America, apparently. Reports from Far Eastern Asia, where toy collecting is also a popular hobby in many countries, seem to suggest that the problem might actually be even worse there than it is in the USA. On the other hand, there is no substantial evidence that indicates that systematic scalping is a common practice in Europe beyond a few individuals. Some countries' legal restrictions for "private" selling of unopened goods in large quantities might be responsible to some degree.