Hi teige,
It would be a concern if they weren’t an authorized dealer for the mattress they were selling (in this case PLB) and were advertising them as “overstock”, “excess stock”, “scratch and dent”, “truck scuffs”, “minor defects”, or “comfort returns” or other similar terms or if for any reason there was no factory warranty. In the case of PLB you can always call them to see if they are an authorized dealer or check to see if they are listed on their retail store finder here.
I would also compare the price and “value” of a mattress to your other finalists or other similar mattresses rather than a regular selling price or advertised price which may be inflated or not representative of other similar mattresses and of course no matter what the price of a mattress … the most important part of “value” is how well a mattress matches your specific needs and preferences in terms of PPP.
Whether they answered truthfully or not would depend on each person and I don’t think there is any generic answer that would cover any specific retailer so you would need to use “best judgement” and if you had any doubts make sure you check the mattress for any signs of use, wear, or damage (see post #2 here).
There is more about flame retardant methods in this article and in post #2 here and post #4 here but the inherent fire barriers that PLB uses (see here) are “safe” yes.
Just in case you haven’t seen it yet … the better options and possibilities I’m aware of in the Chicago area are listed in posts #2 and #4 here.
Phoenix