I own a MacBook Air and a MacBook Pro - both similar but different in use / function.
I was able to pick up my 2015 MacBook Pro 13" with Retina Display on eBay for $650 shipped without a charger.
Ordered a charger for $30 bucks and was good to go! MacBook Pro was in great shape, only 100 cycles on the battery, last model they made before going to the USB-C charger design. Still super thin, i5 processor, 8GB RAM and a 128GB SSD.
The MacBook Air is a fantastic computer as well - a little less power but better battery life, screen resolution isn't as good. But ultra light and easy to carry / use.
I always buy my electronics used or refurbished when possible. It takes a little a patience but the first thing I would do is assess your needs, define your budget, and take your time in shopping. eBay and Craigslist can be awesome if you do your research and know what you're looking at. If not, bring a friend who does.
Make sure you do the appropriate checks or have the protections in place (like a return policy, lapyap, etc.) depending on how you buy.