You can buy btc from market but you cant buy 100k from one person because no one can have 100k btc sigle handedly so you need to collect them before differnet sources and different exchanged
Explain why you think an early adopter could not have 100,000 BTC?
When we started we could mine them for free on our computers using the CPU and at one point they cost $0.10 per BTC to buy them. So hypothetically an early adopter could have purchased say 50,000 BTC for only $5,000 and at 50 BTC per block would have only needed to mine 1000 blocks, at that time easily done in less than a month on a fast computer.