Selling the DPMS off for a loss is dumb. If you could get $600 out of it, I'd sell it. However, you're more likely to get around $450 if I were to guess. For only that resale value... it is worth hanging onto as either a spare rifle or to cannibalize parts off of it.
Save up, and get a "Tier One" as your go-to rifle. If you take a class, toss this thing in the trunk and drag it along. If you drop an extractor pin and can't find it, a DPMS extractor pin will probably work fine. If you get a catastrophic failure due to a faulty round in your primary, haul out the DPMS and finish what is left of the class. Maybe put some BUIS on it and use it as a trunk gun, or take it with you camping or whatever to keep your primary from getting crudded out.
Spare parts are not nearly as good as spare rifles. When you look at the value of the various parts and the flexibility having a spare rifle gives you.... losing $150 is just not the economically wise move. Keep that piece of crap and shoot the snot out of it while you save up for a Colt, LMT, BCM, DD, etc.

