of course there mass bread for profit and sometimes have genetic problems the deaths of your Guppies are more likely to be change of environment or diseased,
in the past 4 months I have bought 1 male and 4 females all doing well I very rarely have problems keeping/ breeding Guppies
Drop acclimated for an hour. Acclimated very carefully... thats one thing.
So, if change of environment, all my fish would be dead, including my delicate fish, like my Farlowellas. My wild caught fish would definitely be dead. Switching them to different tanks, they'd all be dead.
Second, no symptoms, no other fish loss, not even my own current sensitive fish who were in contact with the guppies. Healthy one day, dead the next. No clamped fins, no shimmies, nothing. Just boop, dead.
I have mollies and platies too, they don't drop dead like guppies do here.
I'm not the only one with issues keeping guppies alive. Theyre genetically weak nowadays. Mass bred fish do not thrive at all. Gotta look for people who actually breed good stock. I wish I could get stock from emeraldking, his guppies are fantastic quality.
But, I think OP is right to be wary of guppies locally to them. If they have issues keeping them alive, definitely try a healthier alternative, which are endlers and endler hybrids.