Dear Marcel,
Not sure why but the forum email notification for this thread landed in Gmail spam folder or was never sent. I would have replied earlier.
The problem you reported is due to javascript problem on the page not in QuickSell or you have not enabled any payment modules. There is a fix or refund moneyback guarantee when there's a bug and we didn't help, that is correct, but I am afraid this is not the case. Same version works on literally thousands of sites.
It has happened many times that customers have some sort of javascript issue on the page (and javascript is used to submit the quicksell order, and they blame the issue on QuickSell). The thing is that when such problem exists, the web browser halts the script operation on the whole page. For faster communication, be so kind to submit support queries directly to the support email, because the public support forum takes some time to get a reply, especially on such occasions where there's DDoS or something and the email notifying me you posted to this thread never arrived.
Thanks