It's difficult to imagine that there would be a black market for these types of maintenance medications, but there are a lot of folks without insurance and even inexpensive telehealths will cost ~$40 per visit and would be unlikely to prescribe multiple months of any medication.
When I get an upper respiratory infection, the only thing that will usually only help me is azithromycin. I can't tell you how hard it is for me to get this Rx. Usually it's a NP who is more willing to prescribe it, but if I happen upon a doctor at urgent care, it's definitely a no go most of the time. I'd buy four courses of azithromycin if I could trust the source. This b/s/t market is not where I would get it though. I'm thinking IndiaMart or Canada at this point. I feel hostage to the US healthcare system sometimes... So, I totally get people wanting free access to some of these medications.