This is for the business members of this site - but all can give me their opinion if I'm excessively paranoid & defensive.

I received an email from a company wanting to buy stock from me.
I'm based in Melbourne Australia, and sell premixed coffees - the prospective buyer is Chinese.
They request a price list - I OK this.
They come back with an order of $400k of stock.
I suggest they should buy a sample lot first ~$15k
They are firm - they want $400k... who am I to argue.
I suggest 100% prepayment - they say 50% - we agree on 80%
Now they want me to visit them to sign an agreement.
OK - $3000 for the round trip isn't a lot of $ - but the week spent??? would be OK if my spidy senses weren't tingling.

I've asked for their DUNS#, company registration # ... still waiting.

Is my experience common?
Am I overly cautious? .