Hellos welcome of couse its safe to buy life rocks from reefers
it doesnt mean that by getting from the shop is the best as reefers dose bacterial and stuff into their tank more than those in shops
so if you are gonna get from a decommed reefer should be pretty alright although some reefers still take extreme caution as they do not like pest in their rocks
I dunno how to tell if a rock is healthy all I know if the rock smells like the ocean haha it should be fine