The Outcast Collection Agent

 refers to an unmarked and repeatable quest in Fallout 3.

You can trade some pieces of power armor, (Power Armor marked only as "Power Armor" or Enclave Power Armor. You cannot trade Brotherhood Armor.) some energy weapons, sensor modules, and scrap metal to Protector Casdin at Fort Independence. In return he will give you your choice of the following, in quantities according to the perceived value of the item:
 * 5.56mm ammunition
 * Frag grenades
 * Rad Away
 * Stimpaks

Do not bring outcast power armor with you when you talk to him, as he takes it away.

Once the Lone Wanderer has gained 50 "points" (see table below), they are told that they are considered friendly by the Outcasts. This allows for infiltration of Fort Independence without encountering the hostility of the residents (see Getting In).

Value Table
† Scrap Metal must be traded in groups of two; Alien Power Cells must be traded in groups of twelve.

Collecting for Profit?
If you want, you can make a profit from Casdin by re-selling the items you trade him for. But is it worth it? First, here are the items you can receive, and their value:
 * Stimpaks at Base Value of 25
 * Frag Grenades, Value 25
 * Rad Aways, Value 20
 * 12 5.56mm Rounds, Value 12 (12x1)

As you can see on the Value Table, in any one deal, you can receive an equal number of one of these: Stimpaks, Frag Grenades, or Rad Aways. Or, you can receive a higher number of 5.56mm bullets. For the items besides the bullets, the Stimpaks and Frags are worth more than the Rad Away, so the Stimpaks pretty much win out of these three on a purely profit basis - they're weightless.

The 5.56mm rounds do not scale the same as the other three items. As the number of reward points increase, the better the ratio of bullets to the other rewards. For example, there's a 12:1 ratio at 1 point, but a 24.2 ratio (484/20) at 10 points. However, if you compare the value of Rounds to the value of Stimpaks for any given item, Stimpaks always win. At the low end, you get 2.083 times the value (Sensor Module: 1 Stimpak = Value 25 vs. 12 bullets = Value 12). At the high end, Stimpaks still (barely) win at 1.033 the value (Alien Blaster: 20 Stimpaks = 500 vs. 484 bullets). So, Stimpaks (or Frags) are always the best reward, on a pure profit basis.

If items you collect are in very good Repair, almost none are profitable to "launder" through Casdin. (Barter skill doesn't matter here, because both the original item and the reward are usually subject to barter, even though Casdin isn't.) The only things that at least break even when at 100% repair are:
 * Alien Power Cells (Base Value: 120 for 12), which will show a 2.3x return (275 for 11 Stimpaks) - but these cells are few and far between, and priceless if you want to use alien weapons
 * The Alien Blaster (full Value 500) itself just breaks even (20 Stimpaks) at 100% repair... but it's extremely rare and valuable, plus, who'd repair it with the Firelance, anyway?
 * Finally, 2 Scrap Metals are worth 20 (with Walter's Scrap Metal quest), but get you a Stimpak worth 25. In this one case alone, your Barter skill must be very high (>80), because Walter's 20 caps are not subject to barter, while selling Stimpak rewards is.

Conversely, if items are in bad repair, Casdin still gives his full reward. This makes them worth considering. For example, if degraded to 25% of their full value: However, So weapons need to be in really bad shape to make a profit via Casdin - Plasma weapons should be less than ~20% repair, and Laser weapons should be less than ~10%.
 * You'd make 4x profit on the Alien Blaster at 25% repair (value 125 vs. 500 for Stimpaks), but again, you'd probably not do it. (Unless pickpocketing Casdin - but that's an entirely different story.)
 * Enclave and regular Power Helmets (base value 110; 25% value 28) will make a 2.7x profit (3 Stimpaks = 75)
 * Tesla Power Armor makes 1.6x profit and Enclave Power Armor makes 1.4x when at 25%
 * Regular Power Armor only makes 1.2x when at 25%
 * The Plasma Pistol and Plasma Rifle only get ~.76x (you lose money), and
 * The Laser Pistol and Laser Rifle only get ~.52x (you lose half your money), at 25%.

The Sensor Module is always a bad deal, on a pure profit basis - you're cashing in something worth 30, for something worth 25 at best. But they can be accumulated earlier than the other items to meet the point requirement for Getting In to Fort Constantine, in exchange for grenades or ammo that exist in limited supplies at low level. The 250 cap loss (50*(30-25)) is a small price to pay for early access to the Outcast's interior. Still, Sensor Modules are also an important component of Bottlecap Mines, which are probably the best early money makers from schematics, and pack a helluva punch, too. Maybe Scrap Metal is better for Getting Into Fort Constantine early, but you'll need a lot of it. Your choice.

In summary, is it worth funneling items through Casdin to make a profit? Yes, you can make money - but it's probably not worth all the hassle.

Still, you can use the math to see which items are best for Getting In to Fort Constantine cheaply and/or early:
 * Scrap Metal, particularly if your Barter is high (but you'll need 100! that's 1000 caps Walter's not giving you)
 * Sensor Modules, if you want to get into Ft. Constantine quick and aren't worried about Bottlecap Mines
 * If your Barter is low, give banged-up Power Helmets or Power Armor, or really banged up Plasma weapons, or practically dead Laser weapons.

Bugs

 * Sometimes Protector Casdin can be found Dead, This means no more trading
 * There is no option to give Casdin a Tesla Helmet or Prototype Medic Power Armor.
 * You also cannot give him a Gatling Laser, though this may be due to its classification as a big gun.
 * If you give him the Alien Blaster, kill or pickpocket him, and take it off his corpse, its condition will be restored to 100. If you pickpocket rather than kill him, this can be repeated for numerous rewards and repairs.