Some users can have a fixed amount of additional fields, EAV or not?
Since Consumers and Merchants share some common data as Users they should be in a single base table, as you have already mentioned.

An EAV implementation can be successful under some well planned and controlled use.  But I see nothing in your question that would imply that an EAV solution is the right answer.

In fact, as you move further in your development you might find that not only do Merchants have some additional attributes, but so do Consumers.  If so, you might want a simple Supertype-Subtype model.

That would give you 3 tables that are related as shown below:


               USERS (contains common information)
                 |
                 |
     ____________|_____________
     |                        |
     | (details specific to   | 
     |       a subtype)       |
Consumers                Merchants    



If this works for you, much of the time you would only access the Users table, but when you needed additional information for a Consumer or a Merchant you would like to those details as well.

This is a pretty simple approach and avoids unneeded duplication of data.

For a perspective on EAV implementations, you might look at Aaron Bertrand's post, which explains the advantages and the limitations. 


https://sqlblog.org/2009/11/19/what-is-so-bad-about-eav-anyway


Note that in this case it was rapidly changing requirements which made the EAV decision a good choice.