To read the lot codes: first letter indicates country of origin factory, second and third characters (first two digits) indicate the year of manufacture, so 20 would be 2020, next 3 digits indicate how many days since the start of the year they were made (so 330 would be 330 days since Jan 1, 2020). Last one is the machine that made it.

If anyone can chime in on where that 5 year rating comes from that would be great (I know for a lot of drugs the maximum safe time is from tests done by the manufacturers and in their own interest they only test for a few, maybe 5 years and when the US military tested some substances they discovered they retained most of their potency for much longer). As I wouldn't be surprised if it's a case of 3M only warranties them that long and under proper storage they can last longer but at the same time I understand there is something of a static charge in parts of the mask that does the heavy lifting for particle trapping and that may dissipate over time or other materials may suffer degradation. I know it's important to keep them sealed in the wrapper packaging until time of use for instance.

The things to look for as I understand are: they don't smell strongly off when opened (outside of packaging smelling doesn't matter), the adhesives aren't breaking down (nose pads coming off, melting), the straps don't snap on first use (probably least consequential but it could be a sign of bad storage conditions and stresses to the rubber/elastic and hence the rest of it) and they fit test properly. The straps on these ones aren't that great anyways, I've had store bought ones of this model break on me after the first use if I pulled a little too far trying to get them out and over my hair without tangles.

Which material would be the first to fail? The elastic?

Most pharmaceuticals are shelf stable for a human lifetime, but there is little incentive to do the validation testing to prove it. I suspect masks are similar.

Edit: Never mind, I see you already said the same thing.

It's interesting how YT channels evolve. His first ~140 videos are mountain biking cams.

It's being clearanced. The company would have to throw them out within months anyways as they can't sell expired masks and can't use them and I guess they must have many tens of thousands of these to get rid of. Some of it could be them just not wanting to carry them anymore either, wanting to free up the space but most are probably not that new either. That and probably some weird automated pricing system as even then it does seem kind of low.

These are left over cases from the original pandemic in 2019/2020 that wholesalers basically got rid of and resellers picked them up for next to nothing. When the last batch of these popped up there was a photo someone took of a dozen or so cases piled up to go out to the garbage. I use my box as backup and to give to anyone that comes over to the house. Masks are too expensive to be just giving them out to randos.

Re bulk resellers at 50 cents each

Check other amazon listings, ebay, etc. It's entirely an online phenomenon.

You'll see suspiciously cheap n95s sold by random looking companies or individuals listed as seller and the reviews note they just got a bunch of loose masks not a proper 3M branded retail box so those types have absolutely been buying up this old stock in bulk for at least a year or two now, breaking these 440 boxes down and selling them in lots of 10 or 20 that they toss loose in mailers or amazon boxes and undercutting retail prices by anywhere from a mere 20% to up to more than half off, usually around $0.40 to $0.80 a mask.

Problem is these types of sellers often don't disclose this or make it super clear what you're getting. Plenty of people are fine with it because of the price break, but others are upset to see it. Retail packaged masks seem to sell for around $2 a piece usually, maybe $1.60 or so.