48 hours is the max I do, but they can live 72 off the yolk they are absorbing. That's how shipped day-old chicks survive a two-day trip. You just don't want to open the incubator if you don't have to during the last three days (lockdown when humidity is raised to 60-70% and auto-turner is...
I have bought eggs three times from three different sellers on eBay and had decent success each time. If the eggs are mine, I run pretty close to a 100% hatch rate. With shipped, about 70%.
What seller told you about the 12 hours is what I also do although sometimes I just left them...