It is all a case of demand and supply.
Supply (seller) is perfectly willing to sell at price x which a buyer snaps up. Later on buyer sells at price y (where y>x) with another seller perfectly willing to buy at said price.
It is the way the market and the capitalist world we live in operates, so I don't see the problem in that. The MANNER in which it is done though (Yuringa's case as described) is slightly different due to the usage of lies to get things done his way. At the same time the seller has every right to stick to his or her price cause ultimately, you are the seller.
Last advice, NEVER succumb to the student ploy. We were all students once and I never resorted to using that status as a bargaining chip. If you can't afford it, don't buy it. I saved my ass off for my Gibby throughout secondary and jc and all it takes is discipline.