You know what you're buying/paying
You know what you're selling/receiving
You are at the same airport as the other person
You and the other person are in the Trade menu
Make sure you do not use commas in numbers
You have both sent and received a trade offer
You see the "Trade Successful" and "Data Saved" pop ups

Note: Giveaways and free trades still require the receiving party to send a trade back of a trivial amount of WP/money, usually 0, to confirm a trade.

You are at the airport you want your new vehicle to be parked at
You are in your Inventory (Menu > Profile > Inventory)
You have all of the parts required

See below for crafting kits that require another full vehicle.

The vehicle you are using to craft is at your current airport
The vehicle you are using to craft is currently selected
(If you press Free Flight, you should spawn in that vehicle)
You are in your Inventory (Menu > Profile > Inventory)
You have all of the parts required

Note: Newly crafted vehicles that use another vehicle will change the Original Owner to your current username, but will inherit the flight time and distance of the original vehicle.