Searching gun broker might give you a better idea.
In general a used gun is going to bring 60-80% of whatever the new "street" NOT MSRP price is. If you know what they go for new you can make a guess as to what you could reasonably expect to get used.
Also keep in mind to a dealer who has to make a profit your return will be even less, they are not taking advantage, they have costs that need to be met and expect something in return.
Also keep in mind you have 1 thing going for you and one against --- for you is it's election season and people are a little crazy as they were last year and lots of guns seem to be selling due to this.
Against is the fact the ACR around here at least is not as in demand as the SCAR, HK 556, or LWRC guns. Nothing against the ACR as a gun it's simply not generating the demand or cult if you will desire to own it that some of the other guns have generated.
If I were you put it up here or elsewhere for sale at 80% of what buds is selling them for and take a reasonable offer.