Unfortunately "Imbel" by iteself doesn't really tell you much. Many manufactures built rifles on imbel parts and receivers. The only "real", complete rifle built by imbel were those imported by Springfield Arms as either the SAR-48 (preban) or SAR-4800(postban). Century Arms built rifles on imbel receivers as did IAI (called the M444). Many importers (Century included) also sold stripped receivers - the FAL is a big hobbyist gun. In fact, you'll probably come across more rifles w/imbel receivers that are kit builds vs. built by major manufacturers. As to who built it - who knows? It could have been a professional smith, it could have been Bubba in his basement - no way to really tell for sure.
There isn't really a problem with a home built gun (I've built two myself), if you know what you are doing, but you can't be certain he builder did and they generally don't have much of a resale value. Perhaps he was cheap and decided to set the headspace by taking a dremel to the locking shoulder included in his kit instead of buying the proper sized on. Perhaps he didn't bother to headspace it at all. Did the builder bother to make it 922(r) compliant (i.e. - getting the imported parts count under 10 by replacing parts w/US made ones). If so, which US made parts were replaced, and which manufacturer's replacement parts were used? Is it AWB compliant? If it's an 18" barrel, it was likely cut down - did they take a chop saw to it or was it cut on a lathe. Was it properly crowned afterwards? Etc., etc., etc....
Basically, if I haven't gotten the point across already, there are potentially many unknowns here... Unless it is a Springfield or IAI gun, the origins can be anything. If you don't know what you are looking for, it could be hard to tell if a Century stamped receivered rifle was built by Century ($400), a $1000+ gunsmith build, or Bubba in his basement. Unless you know the origins of the gun for certain or can take someone along who is knowledgable with FALs to look at it in person, you may want to pass...
If you can get more information on the markings on the receiver, perhaps we may be able to narrow it down a bit - importer, any additional model #'s, look for year stamped in the magwell, etc.
Rocko