I have ordered myself few (which I plan to mine), and I have been asked few times if I know whom is a trustable seller, but unfortunately the people I know are miners, not resellers.
Buying a Neptune preorder with BTC, unless it is from your best friend from childhood, should be avoided. That is because you are transferring BTC and only getting the promise of a future delivery. Would you give 20k in cash to a complete stranger from the street and just ask him to promise you that he will send you a Neptune in few months to your home? You don't even have prove that he has bought a Neptune.
Also, KNC has made it impossible to resell a preorder by not allowing the preorder to change owner. For KNC the customer is forever the one who paid the preorder, KNC delivers the Neptune to them, and KNC wont even reply any question about a pre-order unless it is from the registered email of the original customer.
Here an answer from KNC when requesting info about my preorder, however from a different email account:
"If you are the customer that this order belongs to, please use the same email address you used to create this order. "
Escrow to protect the buyer doesn't help either. You will end up paying huge money to an Eastern Europe wallet, and get delivered by DHL a 5kg box of apples (so that Escrow will release the money to the seller, as the DHL delivery took place).
BTC is plagued with scammers. More than 90% of offers in Forums for selling miners are plain fake.
If you want a risk free Neptune, you need to buy it from KNC directly, no intermediaries. Most of intermediaries are scammers.
The problem is that you wont be able to get the earlier batches of Neptunes.
If you MUST buy an earlier Neptune, the safest way is picking it up by yourself (flight, review the unit, pay) and only if it is from a trustable source. If the source is not trustable, scammers might let you review a unit, take your btc and walk away (if the transaction is in a coffee place) or just kick you out (if transaction is at scammers rented office).
If you can wait and buy from the next Neptunes batch (3rd batch, hopefully coming soon), you wont have problems sleeping at night. KNC already proved itself as a trustable seller. And if you wish, you are welcome to get a small rebate
when ordering directly from KNC site (see my signature below).
Libitum