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I think the reason for that is due to the prevailing market rate at the crptocurrency market capitalization at the time someone is buying and when selling. Please try and visit the https://coinmarketcap.com/ . There's actually trading taken place everyday.
I don't know if there are other better answers to this but don't be confused bro.
I think @surpassinggoogle can share more light or anyone else.

Yes, it can vary. You can set the price too, so it depends on that. That's how much the next person wants to sell. This may confuse you a bit more. I am not too good when it comes to trading. Hopefully, someone will visit this comment and help

I believe @prechi or @ewuoso can be of help.

Hahaha the traders

Right. I'll look for them @penauthor