Hello Josie,
Thank you for reporting this. may I know from what version you upgraded sellacious to 1.5.3.
Also always upgrade your marketplace on staging and never on your live website.
Install core first and then extended. this should solve your issue.
If you still find the problem, disable emailer plugin and check if your website is working.
If you still experience issue send us access details and we will look into it.
-Abhishek
Team Sellacious