Sell high, buy low. That"'s basically how the stock market works. How your pension works. How almost everything works. Like it or not!
The stock market is supposed to work on a basis of "creating value" (or, at least, that'"s how the market makers would have you believe it). Indeed, from the end of the 1920s through to the beginning of the 1980s, that"'s more or less how the stock market worked.
You invest in a company, the money you invest is used by that company to create value, and as a reward you get a dividend. If the company is doing well, then there is also the chance that your investment has, itself, gained value, but that was rarely the goal – the "payoff" was that the dividends paid by a company doing well exceed what you would get by simply sticking the money in a bank.
Since the 1980s, though, with increasing deregulation, we have gone back to the good old bad old days of pre-1929 cutthroat capitalism, with the added "bonus" that there'"s nobody with the clout or the money to take on the role of J.P. Morgan. We'"re currently in a state where capitalism has become zero-sum. It"s broken, situations like the southernEuropean debt crisis are merely minor surface indicators of the endemic rot that lies beneath. Capitalism, in it"s current form, will die.
Anyway, criticisms of the financial system aside, the short term model you are describing is no more and no less than "arbitraging" – buying from one market to sell on another, and (hopefully) gaining a profit from the differential between the two markets. It has nothing of "investment".
As Jongoleur points out, greed, selfishness and lack of ethics are attributes which would at least lose you friends in common interpersonal relationships. They might be "desirable" in our materialistic, ever-more greedy model of consumer society (mirroring as it does the broken Reagan/Thatcher capitalist model that underlies it), but that"'s an indictment of society"'s failings, not an indication that there"'s anything wrong with basic interpersonal relations.
So yeah, sure, I understand why the ebay sellers are doing it. But it doesn"'t make it nice.