It does require a second account or having two computers, but I'll post it here anyway.
The instructions have been posted here before, I didn't figure it out on my own.
1: buy a second copy of the game or ask someone to buy it for you if you don't have the money.
2: create a second user account on your computer.
3: enter the registration information you got from the second copy of the game into the account you created.
Though you're probably doing this on your own computer, I'd recommend creating at least a simple password for the second account.
You only have to do these 3 steps the first time you're going to self trade.
After you've registered the game under the second account:
1: create a room on your main account, the one you're going to use to complete the game.
2: switch to the second account and join the room. You can now trade anything you want between the two accounts, or even battle them!
If the reason you can't create a second user is because you don't have a second Microsoft account, I think Windows 10 has an alternative method for making a new user.
If you're using someone elses computer, one that the whole family uses or one with restrictions (a school computer, for example) then self trading would be impossible. Note: I'm not trying to offend anyone who's using a school computer for games or one with restrictions, I was just using a school computer as an example since I knew some students at Perkins who would play audio games on the school computers, and it took a month or so just to get the games downloaded since Perkins puts so many restrictions on their student computers. Well, at least in Lowerschool they do.