Interesting point, if I was able to sell all hardware for the initially paid price I would only just reach the same financial outcome as option A. And obviously it's not going to sell for the brand new price...
Surely if you were able to sell it now you would have:
The 50% of cost deposit which they have paid
plus
Whatever you can now sell it for
Meaning that if you can sell it for at least 50% of the initial price, you will not be out of pocket?
So after weeks of waiting round for responses, I've had this from them:
I'm getting a little bit worried now, there's no way I can refund them the money they gave because it was used to purchase hardware that has now devalued past the amount paid, purely because of the amount of time they've taken to come to a decision.
I certainly recall verbally stating that this was a deposit, but this is not written on a document as such other than the receipt which they claim to have never initially received.
Grumph.
If they had concerns about your initial proposal (which they obviously did otherwise they wouldn't have had "two independent IT advisors" to verify it) then they shouldn't have accepted it in the first place.
They paid your deposit in order to secure your time, effort, and provide you with the security required to go make the initial purchase necessary to move forward with the project. The fact you couldn't afford the initial payment without this deposit is irrelevant - most businesses would require some kind of deposit from a new, unproven client precisely to avoid this kind of situation.
As far as I can see, your options are:
A) Insist on full payment from them for the original contracted amount.
B) Inform them that you will be selling the hardware to recoup your costs, and that they will be forfeiting their deposit as you require it to make up the shortfall and compensate you for your wasted time/effort.
The third option, is that you will sell the hardware, and use their deposit to make up the shortfall, as a gesture of goodwill you would be willing to return any excess once your costs have been covered. BUT make sure you point out this is a gesture of goodwill and you have no obligation to do this as they have breached the original contract.