What he said^^^ I mentioned it as there are plenty of older systems being sold on eBay, Reverb, Craigslist, etc, and some are likely to be "less than legal"(and in most cases, the seller probably doesn't know any better). A quick google search turns up plenty of info, but it may be daunting for someone with little experience with this stuff so here are the big highlights:
Paste from an FCC website page:
Most of these 600 MHz frequencies have been repurposed for new 600 MHz service wireless operations (specifically, the
617-652 MHz and
663-698 MHz frequencies).
For the most part, any system that operates OUTSIDE of those ranges should be okay.
https://www.fcc.gov/consumers/guides...ss-microphones
While this says "microphones", IEM systems use the same technology, so all the info is still pertinent
