the model is for me. I wrote a crude btc trading bot, and I had three major objects: Market(), TradingStrategy and SimulatedMarket which used historical data.

I did not model individual transactions, fees, etc as objects. The idea was that I encapsulated all the stuff that's relevant to a strategy in one object, and the market object would give me things like get_price(), buy(), sell(), which would also be available in the simulated version.