If I for example buy cheap products from china and sell them with a higher price to other people in a different country in ebay - is this really wholesome?
It depends on the intention / volition motivating the action. If the action is based in one of the unwholesome root defilements, namely Greed, then yes it is unwholesome.
If it is based in the wholesome roots of Renunciation, Good-will and Harmlessness, then no, it is not unwholesome. An example could be, that one tries to earn more money in order to donate them to charity or help beings in need.
If one just wants to earn more money for oneself, then you are right, it is not helping either oneself, other beings or society.
Another key factor is the consequences of ones action. One should ask oneself, what would happen if I payed the seller in China a larger salary? It might be more beneficial to pay them more, than to earn more money for oneself. If the seller needs the money more than oneself, then one could help that being by paying more for the goods.
If people have more money to cover their basic needs, then they will also have more time for spiritual / religious practice.