[discussion]skin value loss?

Hello, I have been playing Counter Strike for a couple of years and it is finally time for buying a knife and perhaps some gloves as well. One of the biggest reasons for this is that I know that I can sell them for cash when I am not playing anymore. Sinne I can buy these items at a third party website (DMarket, Skinport, etc) for a relatively good price, how much should I expect to lose after selling them. Do you think skins(especially knifes and gloves) will decrease in value over time(1-5 years)? I am trying to figure out my budget for knife and gloves, because if the loss will be only the fees of the websites, could I then not throw money way above my budget on this? Thank you, looking forward for your thoughts!