How much does the Sentinel trick cost?

You buy it for 500 dosh, deploy it, then sell the deployed unit back for about 325 (the author is not completely sure), so it costs roughly 175 dosh.

Does the Sentinel give XP to Commando?

Yes. When the turret shoots a ZED, Commando gets 50 percent of the XP and your current perk gets the other 50 percent. If the turret only damaged it and someone else kills it, it gets full value.

What are the weight requirements?

You need 3 kg free to carry a Sentinel, so drop a weapon first if you cannot.

Tactics

KF2 Sentinel Trick: Nearly Free Turret for Every Perk

The Sentinel is a deployable turret that any perk can buy. This community tip, written by a self-described Sentinel fan, shows how to get one for almost free before the boss wave or any other wave, and lists practical facts about how it behaves.

The trick

When the trader opens:

Drop a weapon if you cannot carry the Sentinel (3 kg needed).
Buy the Sentinel for 500 dosh.
Deploy (use) it once.
Drop it, or sell it. It sells for about 325, so it effectively costs 175.
Pick your weapon back up and you are ready.

Basically a free turret with extra firepower.

Good to know

A well positioned Sentinel will not backstab or anger Scrakes or Fleshpounds at the wrong time.
If you drop a deployed Sentinel and someone else uses it, theirs explodes yours. So dropping is not always a good idea.
A Sentinel placed where the boss spawns can damage it during its spawning video. A bug once let the author kill the boss before the video ended; it was fixed long ago.
When the turret shoots a big ZED or boss, Commando earns 50 percent of the XP and your current perk the other 50 percent. If only the turret shot it and someone else kills it, it gets full value.
This makes Sentinel a good way to level Commando without playing it. The author took Commando from 0 to 25 and tier 3 to 4 using Sentinel on other perks.
The explosion mostly staggers rather than damages. It staggers small ZEDs and Scrakes, but not Fleshpounds (raged or not).
Sentinel counters E.D.A.R. robots well. It is poor against Bloats and armored ZEDs.
There is a bug where the Sentinel explosion can hurt you, sometimes from 100 to 0. The same applies to the Gunslinger grenade; the author could not reproduce either.
It works on almost any perk: Support, Berserker and Survivalist worked well. It failed with Sharpshooter and Demolitionist because 3 kg is hard to free up, depending on guns.
Not sure what to do with 3 kg? Buy a Sentinel. It increases your DPS and covers your back. Dropping it takes time, so a badly timed drop can lower your damage.

Bonus tip: Medic pistol

Upgrading the Medic pistol is a free 500 dosh upgrade without weight gain, which means more heals.

For more turret and weapon advice see KF2 best weapons.

FAQ

How much does the Sentinel trick cost?: You buy it for 500 dosh, deploy it, then sell the deployed unit back for about 325 (the author is not completely sure), so it costs roughly 175 dosh.
Does the Sentinel give XP to Commando?: Yes. When the turret shoots a ZED, Commando gets 50 percent of the XP and your current perk gets the other 50 percent. If the turret only damaged it and someone else kills it, it gets full value.
What are the weight requirements?: You need 3 kg free to carry a Sentinel, so drop a weapon first if you cannot.