Is there a good indicator of what you can sell to merchants? Ive been collecting a ton of stuff but don't know what I should keep or sell.
Focus on upgrading your resources satchel to carry more.
At the beginning, knowing what to keep is confusing.
Eventually you will find that you want to buy armor and weapons from merchants. So you're going to want to collect metal shards and the special resource the merchant wants for the item you need (and it varies by merchant. Different merchants have different items and different "prices")
You also want to keep the gear/modification things. The strongest ones you can find. These stat-boost your gear.