OK.... thanks for the useful advice from you all.

To be more specific... I need to add that I was looking at the mobile app indicators at that moment, as I didn't have the opportunity to use the advanced functions at the time. It concerned also the hectic period of the Elon Musk twitter influence....
Transaction costs are under control so that wasn't the problem, but I agree that I'd better move to another platform...

On Friday 29/1 at 10.03 CET, I did a BTC selling transaction at Bitvavo.com.
The realtime ticker shows a price of 29042 € at that time.
The transaction was immediately confirmed, but has been processed at a price of 26500 €.
How to explain that big difference? How can I avoid in the future?

Thanks