Will this be a problem?

Hello, i'm gonna be moving to Japan in 1-2 years (yes i know there is still time..) but i am hesitant about something. I will be getting my money from my friend by selling my computer to him and some other things, will cost around 20.000$.
I read it somewhere that they will check your bank account and transactions month by month to see your income. But the money will be in my account in one day since he will give me the money in 1 day after i sell my things to him, he is not in my country right now i will sell it to him once he arrives. And since he is my friend and i'll give him the things from hand, we will not have any legal documents to prove the transaction that i've actually got this money by selling my computer and other things.
Will this be a problem when i'm trying to get my student visa? I have thought about making him my sponsor instead of self sponsoring myself but that seems like a harder path.

On what grounds would your friend be able to sponsor you for a Japanese visa? Is your friend Japanese? More information is needed. Right now all we know about this person is he is buying some of your possessions.

My friend is not Japanese, he is American. Yes he can sponsor me but i thought it would be better if i self sponsored since it seemed like it would be an issue for people other than family members to be shown as sponsors. I just didn't want my money to be shown like a show off money. I have seen some people's visas get denied because of that so i'm trying to figure out a way if there is any way to fix the problem.

Some banks issue certificates for a balance as of a certain date, and others might issue one on an average over a period of time, in which case it might be an issue.

Your visa sponsor would be your school, so you are talking about your financial guarantor i guess. And financial guarantor would usually be a family member with a stable income.

I'ts so sad :-( i hope we will overcome it, because i truly don't know how can i prove that i've sold something to him and money was for that, i understand it sounds sketchy, but i will not focus only the negative. Maybe i can fix it since there is so much time and nothing is happening yet.

Thanks for feedback guys, i'm happy to hear from you and get your help. Have a nice day/night. :-)