(as well as seller doesn't need to be online to be able to offer his goods)
That's already been done for most (if not all) marketplace websites. You probably will need a very inventive, good feature that will attract users to your website; as far as I can tell, your website is a pretty generic marketplace website. You should spend the extra $XXXs to get something that hasn't been seen before.
I was referring to open bazaar, where you have to be online...
Do you have some features in mind? What looks there that has been seen before?