The two easiest ways I can think of are:
If you received the latest Mystery Gift, that gave you Big Nuggets you could sell for a lot of money.
Or, you can collect a bunch of Watts, then going to the Digging Duo, in hopes that they will give you valuable items you can sell. This can get very tedious, though, seeing as you don't yet have the Rotom Bike.
Source: Previous experience and knowledge
Hope I helped!