Back in 2013 (the date he used the platform), the KYC verification wasn't a thing at this time. You could use Bitstamp like in the good old days with no harassment on your ID.
Since 2013 the @OP surely got different IPs and nobody keeps records of what IP you used
So bassacly I have lost alle bitcoins on my bitstamp account because I cannot document how I got the bitcoin? I cannot even rember how I got them.
You could basically say whatever they want to hear. You could simply say you bought the bitcoin from XXXXX. What they don't want to hear is something like "hey, I hacked a website long ago" or "I was selling drugs and I bet all the profits on a gambling site and I won a lot"
If you can't even remember how you got the bitcoins, it's going to be hard. Can't you be sure you didn't just buy them on the platform with a bank transfer? Because you could know where to look at (your bank statements).
The bad news for you is that Bistamp is well known to be silly to verify the customers. They have the habit to ask for deep information like your job etc...
they have frozen my account asking for payment slip etc. for 2013
What else do they ask for? Did they at least ask you for a copy of your ID card and proof of residence?
7
BTC is a lot of money!