What game should i buy on the Steam summer sales for less than 7€?

Maybe you could elaborate and state things like your favorite game genres, etc. so we can give recommendations accordingly.

After consulting your wishlist, I'd say the Witcher 2, Transistor or Deus Ex: Human Revolution. You could also go for Neptunia 2 or 3, HuniePop or the Nekopara Games.

Many of the anime games in your wishlist are heavily discounted, so you could grab a couple of them.
But what's your gaming mood right now? Knowing it might help in choosing...

Age of empires legacy bundle. Unless you already have it or don't play rts of course.

Brothers: A Tale Of Two Sons. Even at full price I can't recommend it enough to peoplet, but at its current discount it's just dumb not to pick it up :D

Crypt of the Necrodancer. No contest.

Life is Strange, Dark Souls 1 or 3, and pretty much any game from the Bioshock series

Rollercoaster tycoon 3 perhaps? xd
or space engineers

From your wishlist I would recommend Huniepop & Huniecam Studios combo.
Or Castlevania: Lord of Shadow, extremely satisfying Hack-n-Slash take on the Castlevania franchise with one of the best boss battles.

Tomb raider

I was going to suggest Bioshock but you got that. My other suggestion would be Doorkickers. I think it's pretty darn cool for some slower pace, yet very engaging mixture of strategy and puzzle, but I'm not sure that's in line with the games you seem to like?

So maybe I'll have to give it a +1 to The Witcher 2.

Which made me realise I don't actually have any of these games, I've only seen or played them with friends :o

Witcher 2
This war of mine
Deus ex HR
Dead space
Dead space 2
Masterplan
Monaco
There are a ton of awesome games for less than 7 bucks buddy