Hi,
When you imported the 300 cars, did you assign them a vendor ?
Vendors can only see and edit their products, if they don't see anything it can be because their is no product assigned to their vendor account.
It can be also some ACL restriction but in your case, I think they are good (otherwise, even if they are users of the main vendor, they won't see any product).
HikaMarket is a secured component, it does not allow vendor to edit products from another vendor.
In HikaMarket the store itself is a (special) vendor with the ID "1", every products non affected to a vendor will be affected to this main vendor.
This main vendor have special rights. It allow you to manage the entire store like in the HikaShop backend.
If you put the users as superuser, them will become a user of the main vendor so they will see all products regardless their vendor.
In HikaMarket it is not logical to let the vendor edit the products of the other vendors except if it is the main vendor which manage the entire store.
So you should put your salesman accounts under the main vendor with just product editing rights.
I will see how it could be possible to add such special feature in HikaMarket but I can't guarantee it because the way that HikaMarket work, it have to propose something secured where a vendor can't access to products/orders/etc of other vendors.
Regards,