The word "built" is the problem. Just because the builder used a Rock River lower receiver or lower receiver assembly, doesn't mean the entire thing is a Rock River-quality weapon. The upper could be some off-brand that has functioning or accuracy problems. Check to see if the finish and color of the upper and lower receiver are a close match; if not, that's Red Alert number one. Look for the stamp on the barrel that describes the brand and/or twist rate, and compare it to the current rifles on Rock River's website. If you can't find a match, that might be Alert #2.
Before anyone could get anywhere close to an accurate assessment of value, I think we'd need to see a couple of detailed photos of the gun and all accessories, get a list of the "performance upgrades" and who installed them, and find out what brands/models of scope we're talking about, here. Two top-end Leupold scopes, or two NCStar import cheapies? HUGE difference. Used "bargain" import scopes are almost worthless in the used-gun accessories market, just ask any storefront gun dealer. Used Leupold optics can still hold half or more of their original value.
Unless it is VERY well done with quality parts (including invoices to show the parts brands, types, costs and sources), and functions flawlessly for several hundred rounds, I'd be hard-pressed to say ANY home-built AR is worth a four-digit price tag right now. My local dealers are selling brand-new DPMS guns for $800 or less, various Bushmasters for $999, and Colt carbines for $1199, and they all have a warranty -- home-built guns don't. Once you own it, any problems that crop up are yours to solve.