Contract law can be relatively simple. The way I see Ellepi's situation, there's a fairly tight contract in place ... documented ... signed by two willing and able parties ... no duress ... clearly time-bound ... with a transfer completed of the all important 'consideration' (a lack of the latter being a very commonly exploited loop-hole when a seller wants to back out of a deal).
I've been in exactly the same position with a motor vehicle transaction. For my deal, I'd paid the seller's full asking price on the spot, and signed off a very tight contract that would see me collect the car later that week. I turned up to collect the car as planned to be told our deal was off unless I was prepared to match another buyer's offer. The seller had already cashed my cheque and when I wouldn't budge, he simply handed me a fat envelope returning my funds in cash. I argued ... and argued ... and argued ... while the seller just stood there stony-faced. Ultimately I walked away, didn't pursue things any further and years later it still annoys me.
Ellepi - obviously you have options available to you including legal channels or simply taking the moral high-ground directly with the seller. You may face a stumbling block based on the fact that the item was significantly down-valued when the trade was completed. Assuming the seller backed out of the trade upon learning he had under-valued his watch when setting the price, he has an opportunity to claim a pricing error that for him negates any intention to create legal relations. Kinda like saying " ... Silly me. Of course I made a mistake on the price tag ..." and sometimes that can be all that's needed to back out of a deal.
And my own moral high-ground ... we all like a bargain but on this occassion, I side with Roger somewhat. Perhaps, everything is now back in order.
This message has been edited by GMTKiwi from IP address 220.127.116.11 on Nov 2, 2017 2:40 PM