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"Just header going to a resonator" could mean a few things... to me, I would HOPE that means you have a straight pipe from the header output all the way to where the normal tailpipe exits the car, but with a resonator section somewhere in the middle.
An exhaust resonator is pretty much a large muffler that goes before your muffler- it's meant to take out / trap undesirable frequencies of sound before they even get to your muffler. Without a muffler it'll still be quieter than just a straight pipe exhaust, but it's nowhere near ideal.
Now what the seller could mean is that there's literally just a resonator hanging off the header and then the exhaust just dumps out somewhere under the car. That would be bad- loud and stinky, plus not healthy for you to drive. The only way to know for sure is to take a peek under the car. If there's pipe going all the way back and you see just one section bulged out in the middle, that's your resonator.