I7 6700(or 7700. or maybe K) price

i7700k, i8700k is best intel for gaming.

Higher is overkill and cost a lot more power consumption, also be sure your motherboard can handle it.

Hmm seems to be somewhere around 220-250€ here in Finland... At least for last year or so.

No more then $100.
You can get a 2700X with 8 cores for about $200 new these days, and when it was in stock Ryzen 1600AF 6 core CPUs went for $80. It's hard to justify spending much on 4 core CPUs these days.

Here are some questions first:
1.What's your PC specs?
2.What's your budget?
3.What games are you playing or planning to play in near future?

How about i3 10th Gen? I heard it's on par with i7 7th Gen

you could get an i3 10100f or a ryzen 3 3100 for less than 100€, both 4c 8t or an 10400f for 140€ for a 6c 12t chip, get a cheap socket 1200 for intel/ am4 for amd mobo for maybe 50-80€ and 16gb gskill aegis for 50€ ... 6th and 7th skylake on ebay is a ripoff

I would guess that he's trying to figure out his options for upgrading without a motherboard change. He's probably got an i5-6600K (like me). Unfortunately, there are no good upgrade options for that socket type. Prices for used 6700K/7700K are stupidly high.

1. The only place you're going to get a "decent" price for either of those is a PC hardware trading forum. You're going to get gouged on ebay and the like.
2. You're still going to pay a premium because those two CPUs are beasts and still very much hold their own (especially overclocked).
3. I wouldn't pay more than $125 or $150 for either. If necessary, suck it up for a while and save for a new mobo/CPU.
4. If you're not going to OC, get the locked 6700 or 7700. They'll be a bit cheaper. OCers love the unlocked versions of those two chips.
5. If you do end up finding one (K) on the cheap, I highly recommend overclocking the hell out of it, if your mobo/cooling allows for it.
6. Neither of those come with a cooler, so be sure your current cooler is up to the task.

I wouldn't pay more than current price of R3 3100 in your region for 7700 (K or not) and even that is bit too much considering lack of warranty and used condition.

I would not pay more than 100€, because for 150€ you might get a cheap Ryzen 3 + Mobo.
Does your Mobo support Xeon CPU? On the second-hand market, the 4c/8t version of it is usually cheaper than a similar I7.