I earned about €16k last year from selling social media usernames but it's a tricky side hustle and would take ages to explain even the basics.
However there's something closely related I can suggest as some form of side hustle. In my line of work a lot of deals, particularly the more expensive ones, are done with Bitcoin. Some of you have probably heard of it, I think we have a bitcoin thread here on NT as well. It's a virtual currency with anonymous transactions. It is also a very fluctuating currency, which is where the side hustle comes in.
It can be a little complicated if you've never heard of Bitcoin (BTC) or how it works.
Think of it as currency trading. As I am typing this, 1 BTC is valued at about ~$833.
Here is a graph that illustrates the 1 BTC's value in dollars over the past year:
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As you can see, its dollar value is prone to pretty significant changes. This is where the money comes in.
I don't actively invest in BTC but many of my business associates do, with good success.
I receive a lot of payments in BTC so I watch the market for a price hike and then exchange my balance for euros/dollars.
For example, a while back I sold 2 kik usernames for $800 in BTC at the time. I waited for a price hike before exchanging my BTC and eventually I exchanged my balance for nearly $1400 when the BTC price shot all the way up.
There are various sites like Coinbase, ... where you can buy/sell BTC by attaching your bank account to your profile.
You can then transfer money from your bank to your account and buy BTC at a good moment (a price drop) and then exchange it for dollars when its price has hiked back up and you're satisfied with the profit margin.
The folks in the NT BTC thread can probably provide you with much better information.
http://niketalk.com/t/302014/official-bitcoin-thread
I would highly recommend a lot of research before diving into the world of BTC investing. After all the price fluctuations go both ways.