Will simple mobile's prepaid sim card work on the iphone 8 they are selling?

I'm thinking of buying a used iphone 8 from simple mobile, but i heard that it might not work and that it might be locked to a different carrier

I'd ask them, but why would simple mobile sell a phone that doesn't work with their service?

If it didn't work,
you would complain to Simple Mobile and get it dealt with,
RIGHT?

It would likely be locked to Simple Mobile. Don't see why they would sell a phone locked to another carrier.

Don't leave the store until you are satisfied that it is working properly.