I agree, it's about making money. This might come as a surprise to you, but people buy your domains to make money from them... be it developing, selling them or otherwise. We don't buy them because they look pretty... I am truly sorry that a domain name I've bought from you is now worth more than when I bought it. I'll make sure to devalue every domain I purchase here on in as not to offend the original seller...
Regardless, you set prices for domains, people offer to pay them prices and then you change them. If I walk into Tesco to buy a £1.30 load of bread, I want to pay £1.30 at the checkout, not be told I have to pay £5.00 because they have already sold a lot of bread at £1.30 today. Your justification is completely ridiculous.
Here's an idea, if you don't want to sell domains at £15 each - don't list them.
Anyway, perhaps you should take some time to consider the bigger picture when conducting business. You might make a few extra quid in sales with your slight price increase but at the end of the day, you've risked your whole credibility as a seller over £60 worth of domains. If a single person doesn't buy a domain from you in future as a result of your actions here then you've cost yourself money.
I won't be opening your threads anymore and I'd encourage any buyer considering one of your domains to not take your prices too seriously.