In most cases, the items they are using as the vehicle for the scam are 1. Large and hard to transport and 2. Used for only one or two purposes and thus are not commonly traded so the scammer get to set the market price.
It is not a scam in that it is arbitrage in name only, but it plays on people's greed to get them to purchase a rare item only to turn around and pull the rug out from underneath them and leave them holding this worthless item.
Personally, I am of the "buyer beware" mindset. More power to the scammers and thieves; they keep EVE interesting.
To expand: just because someone is doing something that entails risk, doesn't mean taking advantage of that is not a scam. In fact, it's the essence of most scams.
Silverstorm said, "Hardly sounds like a scam to me. Arbitrage is inherently risky."
I am having trouble interpreting that like you suggest.
My understanding is that the behavior that was originally called a scam was making it look like there was an opportunity for arbitrage that did not actually exist. That is not just someone bearing the usual risk of arbitrage (second trade happens to be unavailable), but specifically injecting yourself into that window by making sure their second trade will be unavailable by posting an offer you know can't fill.
