Well, I can't 100% answer the question for you because I don't know all the functionalities. It depends on a lot of factors. Do you work with different teams on the Core and the Seller Front? Do the Core and the Seller Front have a. lot of data in common?
In general, I would probably try to keep everything as simple as possible. That means everything in one application.
If you already know it's going to be a very big website, it might be better to split the core and the seller front. You can however, keep using the same database. You can then, for example, build a package that shares the models or you just implement them per application.
Like I said before, it's hard to answer your question without all the information