As a seller myself, I would find it highly unlikely that any seller, for any reason, would knowingly send a malicious file. Moreover, if someone was to do so intentionally, I highly doubt that they would do so in such an obvious way. My personal feeling here, is that if you have received a malicious file from a seller, it is likely by accident.
As for how this could happen, sellers like me receive and send files all the time on Fiverr, so it’s not difficult to imagine that one or a few of us could unwittingly distribute something malicious at some point. In fact, I use Linux. In this case, my computer will never be affected if I receive virus via a malicious file. However, it would still be possible for me to pass such a virus on to a Windows computer without realizing.
Best practice in this case, is to always scan any received files prior to opening them. Also, in your specific case, I would contact the seller in order to ask why they sent you this file and explain your concerns.
I hope this helps.