Is this because it is cast, or because it is cast by Ruger?
You should have been able to tell it was cast for all of the markings (sans serial number) are raised instead of impressions. As for cast by Ruger -- they are supposed to be the best in the business.
Now if you got hosed buying a pre-ban Eagle Arms, but instead got a cast J-15 E. A.Co. with an EAxxxx serial number,
www.ar15.com/content/legal/serialNumberList.html would have told you it was not an Eagle Arms. (Not trying to Monday morning QB.)
You may have some legal remedy if the sale was recent, and the seller misrepresented the product.
There is also still a market for pre-ban lowers in the states with non-expiring AWB's.
Some may deride cast lowers, but unless you intend to use it as a club, how much less abuse one would take than a forged lower is largely academic to most AR shooters out there. (I've heard all the arguments re clearing jams by beating it on the ground, K-booms, etc. but the only cast receiver I've actually seen break was due to the carrier key repeatedly smacking against the receiver extension threaded area, and the idiot didn't investigate when it was pointed out to him
before it broke.)
My first AR was (is) built on an E.A.Co. lower and I have not been overly kind to it; My buddy has several from when they were available for $60 ea. and uses one in competition shooting and load development.
Both have held-up without a problem. I have many (read as "all my other ARs") forged lowers, and I'd prefer $80 for a forged lower from Ameetech Arms any day, but I don't feel handicapped or under-gunned with my Essential Arms lower at all.
If you got a decent deal on it as a functioning AR, pre-ban legal in all 50 states, no worries.
Shoot it and have fun.
Cheers, Otto