Stripped Imbel uppers are going for 650-950 on GB. Last Imbel kit went at 935 on GB. Plus assembly cost and US parts (if any were used).
Built rifles run 1500-2000 range.
It has been true a while that built rifles often are worth less than the separate parts and receiver value with FALs.
I would check headspace, but it is easy as hell to fix if needed, just change the locking shoulder. So if it is functional, I would not over worry about it, especially if you can do a rudimentary inspection of fired brass from it, or check whether a live round with tape on the back goes into battery or not (you need to know how to check the HS on an FAL first, but you might be able to sort of check it without gauges, then actually gauge it later).
Barrel wear- well who the hell knows what it really is- the gauge the seller is using is likely for a .308 M14 or .30-06 M1 type rifle. Unless he knows exactly where a NEW Imbel barrel reads, he is likely full of it claiming it reads as 0 on the gauge. You might be able to find some info online about it, but I can’t recall exactly how most surplus Imbel kits gauged. But I have seen a “like new” kit with a “gauges near new” barrel show up with a well worn barrel that actually gauged as well used, despite visual inspection, so beware there. I have gauged a lot of FAL barrel but can’t recall off the top of my head where to read a gauge line at on the FAL.
Some guy on here flipped out over a SCAR 16 barrel gauging as “half worn out”, the idiot was using the gauge wrong, and the first 15 replies never even considered the fact that he was using it wrong, so that is a misunderstood thing for sure.
Most Imbels will have pretty good barrels in general. (Their ammo budget must have been lacking for training.)