Choose a group that does adventure trips for charity, like hiking kilimanjaro say, where you'd need to raise a fair amount and then do it.
The point it to make sales a "game" for pocket money. You'll never make a ton, but you'll gain valuable experience about writing marketing blurbs, interacting with buyers/sellers, negotiation, the time value of money and products, etc.
As an example: there is, right now, a perfectly working portable air conditioner on Craigslist for free. If I felt like picking it up and storing it until summer I could easy get $100. The seasonal inverse is true for Christmas trees.
...etc etc etc...
And in case you want to get familiar with the profession, I could offer to try Talk to Ri app in about one week) (it's the product I'm working on, it hasn't launched yet though and its content isn't updated for now). https://itunes.apple.com/us/app/talk-to-ri-business-english/...
It's actually more for learning English for tech, but we will have a conversations trainer, including tech sales manager program)
The art of selling today is evolving. On this subject, John Jantsch is brilliant > https://www.goodreads.com/book/show/21847226-duct-tape-selli...
That said, the best way is to practice selling stuff.