My opinion is that the market price of the watch has not gone up by 15% in 13 months. I suppose you have bought it at a discount to its market price and have sold it at market price. There is obviously no rule here stopping you from doing this, but that doesn't make it decent behaviour.
I don't think you are much better than Viperstripes. You bought something which has been offered below its market value, but when you sell it you mark it up claiming that it is still a fair price. Same with Viperstripes: 3k is a fair price for the Rolex Explorer. But as he bought it on the cheap due to forum members not wanting to mark-up this very special watch, I suppose decent behaviour would have been to donate the profit to charity. In your case, I would have found it decent if you had put an equally large discount versus today's market value as you received a year ago or at least would have shared the profit with the person owning the watch before you.
But then different people will always have different points of view about what is decent and what isn't.