Gauges need to be checked annually, and that doesn't matter if the canner is new or used so that's a wash when considering new vs. used. It's easy to see if a seal is working or not, because steam will escape around the lid if it's not. If no steam escapes and the pressure plug pops up like it should, then you know it's got a good seal.
I've heard that too that you shouldn't buy a used cooker or canner, but I haven't seen a good reason why not yet. All of mine are used except my electronic one, and they all work perfectly.
Not criticizing you at all, just sayin' in general how I've looked at the reasons not to buy used.
I know, right? Can't believe everything you read on the internet!
I have three old Presto canners from the 50s, the 7b and 21b, one was my grandmother's. Also 2 Presto pressure cookers and a Mirro pressure cooker/canner. I just can't help myself when I see them at the thrift stores for a few bucks.
These things don't just "blow up", just as guns don't just "go off".
I will suggest, if buying used, to get a unit that still has parts available. And don't buy some oddball from the turn of the last century or something. The popular post-war canners made by National Presto, Mirro and All American are the big 3 AFAIK, and still have gaskets, weights and safety plugs and gauges available. At least the ones that I have do. If the rubber bits are brittle replace them. But if they leak they won't build pressure. They won't explode. Also make sure you have the rack or metal plate to keep food or cans off the bottom of the vessel, or come up with something that does that.
Most of the problems I read about relate to cooking with the pots too full, like a pot of beans boiling up and plugging the weight stem. Then they over-pressure and pop the rubber plug and "explode" beans all over the ceiling. This is not the same as "explode" like a bomb. And 15lbs pressure just ain't that much!
I would also recommend having a canner with a jiggle weight, not a gauge only. The old Mirros had a weight that had 3 holes in it and was used for 5, 10 and 15 lb. The Prestos like mine had a gauge only so constant babysitting to keep constant temp was necessary to keep constant pressure. But I converted them to the new 3 piece weight set 50332 using a new stem 1058. So now the weight keeps the pressure constant and I also don't have to get the gauges checked every year or replace them with equally inaccurate new gauges for $20 each. (been there, did that too)
I like the heavy cast aluminum old American made cookers. The new cookers have silicone gaskets which are nice, and even more safety features that lock the lids into place when under pressure. We live in a litigious society. But that doesn't mean the old ones are unsafe.
But then again, some folks can make anything dangerous, and they shouldn't be around a hot stove in the first place.