If you want to give the magazines away, I think it is reasonable to ask for the cost of postage if you decide to put them in the "free" section. A lot of magazines will fit in a medium flat rate box. ($13.45)
This is some info I found
rhttps://community.ebay.com/t5/Archive-Shipping-Returns/Can-vintage-magazines-be-shipped-as-USPS-Media-Mail/qaq-p/9229231
As far as I can tell, magazines are not in the media mail category.