Offerup, LetGo, Craigslist and Facebook marketplace = some success (free, no shipping)
Forum For Sale/Trading boards = some success (free, but usually have to ship)
Reverb = very good success (3.5% fee)
Ebay = greatest success (3.5% fee for all guitar and bass categories and subcategories to stay competitive with Reverb!)
You literally reach thousands more potential buyers on Reverb and/or Ebay than through any of the "free" offerings. Depending on the nature of the equipment, your willingness to ship it and how much effort you want to put into posting and keeping up listings with all the "free" options, 3.5% is very little to pay for improved exposure and a streamlined selling process that gives you access to people other than "gear traders" or those with sufficient cash on hand.
Just my two cents...