If an ex "blocks" your messages across various services, do you still have an obligation to notify them before you get rid of their stuff?

Tit for tat. 

I would say no. feel free to trash it, burn it, pee on it, give it to Halloween kids, distribute it over 5 miles of sidewalk, whatever you want.

Well almost anything. I guess many places have laws against littering. This post was edited by JakobA the unAmerican. at October 24, 2017 10:08 AM MDT

Certified mail

Is the stuff worth much like saleable items?

If your ex is a complete douche, legally he could go after you and win a judgement against you, if you can't show you've made an effort to return his belongings.  As Thriftymaid states, send him a certified letter.  Keep it simple and businesslike.  Tell him: 

"Since you've chosen to block me on your social media accounts, please be advised this certified letter shall serve as legal notice for you to pick up your belongings within 10 days of the date of this letter.  If I do not hear from you by (insert date here), I will consider your property abandoned and discard it as I see fit. 

Thank you for your attention in this matter.

Sincerely,
JA