Why is it legal for individuals to sell their diabetic test strips?

I past several signs today tacked to telephone poles that said “We buy diabetic test strips.” and included a local cell phone number.
I looked online and found many ads like that. Here is an example.
Are people getting these strips for free as covered prescription items and then reselling them to the buyer so they can make a profit at taxpayer expense?
Why is that legal? How does the second buyer know the strips were always kept at the right temperature and stored properly? Weren’t these items prescribed to help people control their diabetes? I live in NY State. Is this even legal?