whats mean “kauf´doch”? I dont understand why use the ´ and the meaning of “doch” here.
“Doch” is simply used to strengthen the wish here.
Typically used together with “bitte”. “Koche mir doch bitte eine Suppe.”
First I didn’t read where you have the two words from.
If you say “kauf’ doch…” if the person doesn’t do it for you it would translate as “just buy…”,meaning “do it, there is no risk”.
If you want to be rude you can also use the word “doch” to express that you are not affected by what the other person is doing.
e.g. a line from a song: “Spring doch vom Balkon” whose bottomline meaning is “get out of my life, I don’t care anymore”
And you can use “doch” in order to nudge sb to do sth he/she doesn’t want or dare to do. “Spring doch, du wagst es ja eh nicht :-P”
I think this is more than you asked for, sorry 
Great to see someone is learning my language, wish you success!!