Let's say there is a new development under construction. The pre-construction (contract) price for a unit is clearly 1,700k. Those who purchased pre-construction are now offering to sell their contract for 200k, 240k, 350k, etc. These are prices for units of the same size within the same building. How to interpret?

Are they saying, for example, a buyer must pay 350k just to transfer the contract (at full 1,700k price) to the buyer?

or

Are they saying, for example, a buyer must pay 350k to transfer the contract (at price of 1,700k less payments made to date) to the buyer?

You pay the vendor an agreed amount and you pay the developer the contract price of the unit that the vendor is selling, minus anything the vendor may have already paid, plus some fees. You will also be liable for the final transfer costs.

So say a vendor signs a contract for a unit at launch at 1MB and pays 100KB in deposit and instalments. A year later the developer price for the same unit might be 1.5MB

In theory if the vendor sells you his contract for 400KB he makes 300KB (400KB less the 100KB he has already paid) and you make 200KB because you get a unit supposedly worth 1.5MB for 900KB + 400KB.

In practice of course there are fees to take into account, and of course the above calculation doesnt take account of any discount on the current list price of 1.5MB that you might be able to get.

As completion day approaches these "flippers" may become more and more desperate to sell as few of them actually want to have to pay the full purchase price to the developer (many dont even have the full purchase price available anyway). So I would wait until they are sweating a lot before making a low offer. Bear in mind that nearly all these "flippers" are devoid of any merit and have only bought the unit in the first place in order to sell it at a profit before completion. I certainly wont lose any sleep over them if they all go bankrupt one day.

Ok, so it works just like back in the World. The price is for the right to hold the option. Thanks for the clarification!