Credits are useful late-game for rerolling armor abilities and buying weapons from vendors. These can get pretty expensive, though - 30k credits per reroll, or 500k for one of the high end weapons. How can I make the credits I need to afford this stuff?
Start farming the darkzone. In one DZ run, if you extract 30 purples, you can sell them for ~75-100k credits. You also will gain DZ credits and experience, which will put you on the road to the DZ level 50 merchants that sell high-end, ilvl 31 blueprints for DZ credits. Personally, unless there is something you want to buy from the advanced weapon vendor, I would recommend you dismantle extra gear for the crafting materials.
You should also do the 3 dailies (2 hard, 1 challenging) for bonus phoenix credits. You'll definitely want a group of 4 for the challenge mode mission. If you have a regular group, you can repeatedly farm missions on challenging difficulty. This will yield extra phoenix credits, as well as the chance that the boss drops a high end item, or at least a few purples.
Just sell some items you have found and dont strip all of those (i.e. sell green/blue items, Strip purple/yellow ones). Be careful while rerolling yellow items, they will use Phoenix credits and not normal credits!
You can unlock the Darkzone merchant in the main base to get dark Zone items without visiting it ... but the requirement is dark Zone rank 30. So in the endgame you will have to visit the dz for the real cool stuff (yellow items). As described in other postings, scavenging (magicfind in other games) will increase your Chance to find these items.