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Presumably, I want to buy Bitcoin in person somewhere in Europe. I need very big amounts, say, 1K BTC, and I am willing to pay in cash (EUR).

When I find potential sellers online or through friends, I ask them to prove they have access to these amounts of BTC by sending me 1 satoshi from their accounts or by giving me some other proof. However, they all say that they don't just want to show their BTC accounts as it's unsafe and ask for a video of my cash, which I don't want to provide first to anyone.

How should I approach it? How are these trades usually done?

  • sounds dangerous. You could ask them to sign a message with their private key so that you can proove wheather they have the control of an address, so that they don't need to send you satoshis ( bitcoin.stackexchange.com/questions/3898/… ) If they want to offer you 1k bitcoins, then they should send those 1k to a few addresses and proove that they are the owner. Everything else is dangerous for you. Don't let them scam you. – ndsvw Feb 25 '18 at 18:33
  • You may want to read Joseph Wang's Back to paper and gold. – Rodrigo de Azevedo Feb 26 '18 at 3:42
1

For large purchases, you should look at OTC (Over The Counter) Bitcoin markets. Also known as "dark pools".

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Buying that amount in person by cash is totally dangerous, you should buy that amount as parts.

if you are buying that amount online, if the seller is really a "bitconer" and he should know how to deal with multi-signature transactions.

Multisignature (multisig) refers to requiring more than one key to authorize a Bitcoin transaction. It is generally used to divide up responsibility for possession of bitcoins.

Standard transactions on the Bitcoin network could be called “single-signature transactions,” because transfers require only one signature — from the owner of the private key associated with the Bitcoin address. However, the Bitcoin network supports much more complicated transactions that require the signatures of multiple people before the funds can be transferred. These are often referred to as M-of-N transactions. The idea is that Bitcoins become “encumbered” by providing addresses of multiple parties, thus requiring cooperation of those parties in order to do anything with them. These parties can be people, institutions or programmed scripts.

Anyway, the skimmers around are much more than the real sellers, so you can't trust anyone. if you found someone, start by buying low amount and then you both high up the amount.

One more thing, You should wait the transaction to get at least 3 confirmations. Read more about Irreversible Transactions

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+50

This is not only dangerous for you, but also for the people creating a transaction with you. There is money laundering rules everywhere, and people would have to explain, where the huge amounts came from. Governments are chasing tax issues, and investigation services look for frauds of all kinds.

And then there were too many stories of African princes...

So, if the money is legal (clean source), you can easily go via exchanges. We as a community certainly won’t help in illegal activities. If it is not, then there is a major risk, that is not worthwhile to be taken. Scams are all around the corner, and prosecutors as well. Bitcoin is maybe secure what concerns transactions, but anonymity an der privacy is not given per se.

Oh, for a little amount of 3-5% I might get interested helping to achieve your goals :-) Did I just say that?

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Try using www.LocalBitcoins.com. They have designed the website to facilitate cash transactions via an escrow service. You can also see people buying and selling Bitcoin using cash, based on their location, in which you can filter those nearby. I would recommend doing a couple of bank transfers exchanges on the site first so you get a feel for the escrow service and you can also build up a little reputaion to build trust.

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If a bitcoin address is the recipient of 1000 BTC (it is a utxo), and you send 0.00000001 BTC to someone, the transaction reveals the amount of bitcoin on that address, that address, the public key, and the remainder is sent back to the sender on a new address as change (minus transaction fees), a new address which has not revealed even its public keys yet, so no risk.

Many wallets will pick the smallest utxo to send from so if the wallet has received two transactions one for 1000 BTC, and one for 1 BTC and the wallet owner sends you 0.00000001 BTC then probably you will not be able to verify that they, in fact, have the 1000 BTC that they claim as the smaller utxo would be used.

https://localbitcoins.com/ is the website I was thinking of on which may people directly trade Bitcoin.

As others have said, you need to make sure that the BTC transaction is well confirmed before considering it final.

You might also consider using an escrow service.

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