Hello, and welcome to Mayo Connect. We can only share our own experience hare, and not speak for all of Amazon, of course, but many of us have gotten our saline on Amazon when it was not available locally.
Some of us may buy supplements there as well.
Here is my personal guideline for safe purchasing - I look for products sold by Amazon, or by another larger vendor through Amazon. For example, the Naveh brand and Base Laboratories brand are both available to me through my pharmacist, so I am comfortable ordering on Amazon as each company is listed as the vendor.
There may be others, but I stick to those I know.
Sue