When it was new it was a good OK skimmer, but not the best you could get.
If the skimmer is in close to perfect condition, and clean I'd say the price is right, give it a try. If it's got a lot of glop and gunk inside it or damaged in any way it's not worth the trouble.
I found that they were tough to clean because you can't remove the insides to completely get to everything.
While I think the manufacturer said it could be mounted externally, I recommend you only use it as an in sump skimmer. It's a little prone to leaks in some places.
Consider replacing the output hose barbs with rigid pipe, and vent that so they will run at a constant flow. The original air line vents tend to clog.