All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible to sell this chain at the price that you mentioned or at a greater or lesser price on the condition that the sale is hand to hand in one session [in the same sale contract]. The fact that the market price of the chain is £ 300 does not have any effect on its sale price.
It might be that the reason why its price drops is that jewelers do not pay for the price of manufacturing when purchasing the gold; rather, they buy it as raw gold, unlike when they sell it as they charge for the price of manufacturing.
In any case, there is nothing wrong about what you mentioned provided that the sale is hand to hand. For more benefit, please refer to Fatwa 82096.
Allaah Knows best.