Hello, @DerpyOnion (I love that username btw lol) Welcome to The Bell Tree! Assuming you're playing ACNH, and not another title, Nook's Cranny is the place to go for buying, trading, or selling in-game items. The Airport is for visiting other islands with NPCs visiting or other events that people may choose to hold. Hope this helps some!
Hello and welcome, @DerpyOnion! I would suggest giving our Frequently Asked Questions a read. It covers everything you need to know concerning how the forum works.