First you say "Security deposit amount: 6500 EUR or USD" then you say "I, The FuzzStone, will never ask you to transfer any money to me!"
And?

And people are asking legitimate questions about your offer. Being sarcastic and arrogant is likely to prevent you from making a deal, while being open and transparent may help.

Our people are meeting with the buyer, taking his insurance deposit and leaving him document/receipt for this amount of money.
About "running away". The people I work with have an excellent reputation, they have been working on the OTC-market of Ukraine, Russia and Belarus for many years.
I have no reason to doubt the legitimacy of this proposal, but this statement above gives me the willies. All business propositions have inherent risk, and it's usually expected that both sides enter an agreement willing to assume some of that risk. Here it looks like the potential buyer must assume all the risk, and the seller wins either way. And the concern of "running away" still remains. I must take your word that these people (whom I don't know) have an excellent reputation (which I can't confirm.) So I'm expected to trust this "document/receipt" is going to prevent people I don't know, with a reputation I can't confirm from running away with $6500 of my money? What's so special about this "document/receipt?" Does it include the seller's KYC?
300 EUR? That's is really funny.

I'm really wondering how you imagine it. Our people will travel to Portugal for 300 EUR? And if the deal is disrupted by the buyer? And after Portugal, a schoolboy with 300 EUR will offer a deal in Greece, and what, go to Greece? For 300 EUR?
If you can work with such conditions, at a minimum of 20k USD/EUR, without KYC/AML/proof-of-funds -- then I am truly happy for you.
By this statement I can conclude that you expect the potential buyer to pay the wages of your people whether a deal is reached or not. Am I correct?