I want to sell my old used PC parts, but i have no idea how much i should ask.

[MeIiodas]

So as the title says, i kinda need help on what i should ask ish, Im from belgium as well and im the first owner of all the parts. None of them ever got overclocked

i3-4160 3,6 GHz + stock cooler

Asus AMD Radeon R7 260x 2GB 2GD5 (http://)

Crucial Ballistix Sport 8gb-1600

MSI H81M-E34

 

[The Paladin]

well on the use market you can acquire your parts in the approx price range.

i3-4160 - 40-60USD
AMD R7 260x - 50-75USD
MSI H81M-E34 - 25-50USD
RAm about 40 USD

 

[Aeacus]

If you were to buy your components as brand new, here's how much you'd be looking at, except GPU, that has used price:

PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Core i3-4160 3.6GHz Dual-Core Processor (€121.72 @ Amazon Deutschland)
Motherboard: MSI - H81M-E34 Micro ATX LGA1150 Motherboard (€55.00)
Memory: Crucial - Ballistix Sport 8GB (1 x 8GB) DDR3-1600 Memory (€63.99 @ Amazon Deutschland)
Video Card: Asus - Radeon R7 260X 2GB Video Card (€112.00)
Total: €352.71
Generated by PCPartPicker 2018-08-27 14:31 CEST+0200

But since none of your components are brand new, price reduction from brand new price would be at least 50% if not more, and that depends on buyer too.
In other words, you can ask up to €230 from your components. Though, realistically, you're looking €100 or a bit more from these components, IF you can find a buyer for such a low performing and quite old hardware.