DPMS cast receivers have not had any "KABOOMS" that I have ever heard of. In fact, no lower has ever had a KABOOM. AR KABOOMS usualy happen due to overpressure in the bolt/barrel extension area. And it won't matter if the receiver is cast or forged, you will still pick it up in little pieces. See "AR15 Kaboom" on the web for 3 very interesting stories. 26 tons of pressure doesn't differentiate between cast vs. forged.
That being said, the lower of any AR is not a very stressed part. The areas receiveing stress the most on a lower are the hammer and trigger pin holes. There was a very good post with pics here awhile back about a DPMS Cast lower which cracked when it was dropped. Very interesting read.
And why buy from some jag-off who has no idea what he is selling??? Buy yourself some piece of mind and get a new one that is forged.