Hey there;
The easiest way to store posts and coding you don't want to lose is to either save them as drafts (left button at the bottom of the post editor), or as private message draft (you can even just send a PM with the code to yourself, whichever way is more convenient for you).
You can access your drafts
here and your PM drafts
here.
Drafts can be edited whenever you want, and you can simply delete them if you no longer need them.
Just remember that if you load a draft and then post it (even by accident), it will delete your draft of the post, so if you ever end up doing so and want to delete the accidental post, make sure to copy the content again first c:
(Loading a draft loads it into the board you started the post in originally, so imo the best way to store drafts is probably to create them via responding to your trade thread or something similar, so that if you do accidentally post them, it's no big deal.)