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?
I am not familiar with that exchange, but I'll try to help.
First, I think that you've been using a service that does the exchange on your behalf.
It's unclear to me what you meant by "realtime ticker", was that the price they did show in
the "you receive" box (see step 3) or was it somewhere else?
If it was the price from inside that box, minus the fee, then you better ask their support (!).
If it was the price from somewhere else, another exchange or maybe preev.com, then I think that there could be an explanation.
At least I've encountered services doing this:
As I started, they sell on your behalf. This means that they take care of everything, but they don't do it for free.
At an exchange, they have found a buyer. They sold, they probably sent the money to your bank account and paid the fee.
And obviously they've taken their own fees for the service, fees that are usually visible as a bigger spread (sell much cheaper than you'd do, buy more expensive than you'd do).
Of course, you'll have to confirm for yourself that this has happened.
How can I avoid in the future?
People have suggested this and that exchange.
"Normal" exchanges will let you place the sell order which will stay there until a buyer is OK with your price and buys. You'll sell then at the price you want .. or not sell at all.
Or you can sell at the price buyers offer, which may be a little bit lower. This will mean some fee, but very small, about 0.15%-0.25%.
After selling at something close enough to the market price, you'll have to handle the withdrawal and the related fees.
All in all, there are multiple types of exchanges and multiple ways to handle the selling/buying. Be more careful with what you are doing, make screen captures (I am still doing them now and then) since they could help when you are asking here or the support. Read more details. Gain more experience. And until you have the experience try to work with smaller amounts/values.