It's fine. You end up with more money than you can spend, and you'll be leaving loot behind by mid-game.
Here's some tips:
-aim for low-weight, high-value stuff (no crowbars, no rathound leather, no sledges, etc)
-leave cheap poor-condition (as in needs repaired) guns behind
-if what you're selling is worth more than what a merchant has, buy their bullets and/or batteries which you can always resell later
-use repair kits on expensive items that need it, then sell them
-if you can make repair kits, recycle bad items to repair good ones
-always pick up bullets; they are the best loot, medicine and electronics are great as well
All merchants restock every 90 minutes in real time. If they restock and you enter their map, their inventory is set. If you reload from earlier and do it again, it'll be something different; it's randomized.