Trying to sell my AR-15 with a LMT Lower (with 2 stage trigger), Aero 20" A2 upper with RAS Handguard + a TA11G ACOG (3.5x) + Plastic Rifle Case. What would be a 'fair price' for this?

Whatever you want to list it at and someone will buy it for is a fair price these days. If you were asking me what I would list it for, that depends on what I spent on it, if I need it, how desperate I am for the cash price it would bring. I didn't pay anything for it, I don't need it, but am also not desperate for the cash, so maybe $500? If it didn't sell, then I would start dropping the price until it did.

Thanks!

In the end, it’s obviously your choice regarding your asking price, but considering that a new TA11 goes for just north of $1K, the suggested $500 starting price is a bit low. Sight unseen, I would list everything mentioned above at $1,000 and go from there.

Thank you!

I agree with this. I would start north of $1k for what you have. Take some time and read over other postings for AR's on this site and check Armslist and look at prices there. The AR market is pretty good for sellers right now. The only issue you will have is there aren't as many buyers for 20" rifles as there are for 16" rifles. Good luck!