@Partnerprobs You have done nothing wrong OP. Well done on your good fortune!
Puts me in mind of an occasion from some 15 years ago, when my husband bought something off a man at work, (a 'collectible' item,) for £200. He sold it 2 weeks later (to a dealer,) for £700.
DH was daft enough to tell him, and the man went batshit. This man called him all sorts of names. Liar, cheat, and scumbag, were 3 of the nicer names he called him. DH said he paid for the item, he gave him what he asked for, so it was then HIS, and he could do what he wanted with the item.
It caused such a bad atmosphere at work, and working with this man became intolerable, that in the end the other man was put onto another shift, so he and DH didn't work together. He blamed DH for being the reason he was moved, but it was this man's behaviour that caused him to be moved.
The moral of the story is, if you make money off something you bought, DO NOT tell anyone! 
Also, I know several people who buy from charity shops specifically to sell it for a massive profit on ebay. One woman I know has made about £600 in the past 3 months alone. One item she bought was a pair of Converse trainers for £10 from the charity shop. Sold for £40 online. Nothing wrong with it. You've got to make your own luck in this life.