Seems to me: Gross gain was $20.00 Net gain (after expenses) was $10.00. That's the financial outcome. Doesn't matter if it was the same bicycle or not.
That's also just my opinion.
i think i can answer that one. it's the same as buying 100 shares of XYZ stock. you only make or lose money when you sell the stock. so, there's no money lost when he buys a bike back for $10 more than he sold it. he only loses money if he then sells it for less than he paid for it. so, if he buys it for 70, then sells it for 60, he's lost 10 and financial outcome is break even, wiping out the 10 he made when he bought for 50 & sold for 60.