Who buys the 'Stranded' items?

According to this guide, there is a merchant in Meridian who will trade these items for a modification box.

Fun fact: the 3 "Stranded" items are actually a Death Stranding easter egg!

Having collected all three Horizon Zero Dawn Stranded Collectibles, players can take them to the Mysterious Boxes merchant in Meridian and trade in for a Mysterious Box. The box grants players the following items:

• Special Modification Box – rare weapon or outfit modification
• Modification Box – random weapon or outfit modification
• Shard Gambler’s Box – metal shards
• Warm Socks – can be sold to merchants for 100 Metal Shards, it is unclear whether this has any other use or not

The merchant can be found here:

Source for image