OP needs to read "The Life-Changing Magic of Tidying Up" by Marie Kondo.

You don't need to spend $300 to sell an old bike (???). People sell old bikes all the time. You'll get less for it, but more than if you paid someone to fix it up and then sell it.

Packing it into separate boxes is the worst possible idea. Best case scenario, you find all the boxes 20 years from now and get rid of the whole mess. Worst case, things get separated and you find the components after you've already gotten rid of the frame, etc. If you pack it into boxes I guarantee you'll never ride it again.

I'll say one thing I forgot before: RIGHT NOW, remove the stem and seatpost. Put grease on them before reinserting them. If you store the bike in a dry area, they should not become frozen (making the frame and fork worthless) for many years.