There are more than one seller + verb form?

My question is which sentence is correct?

1. There is more than one seller claims that ...
2. There are more than one seller claims that ...
3. There is more than one seller claiming that ...

According to the accepted answer in this question, it seems that using there is is correct. However, I don't know what is the suitable form for claim.

The correct answer is #3:

There is more than one seller claiming...

First off, seller is still singular in this case.

According to this site:

When “more than one” modifies a singular noun, it goes with a singular verb: “More than one person is going.”

Then you have the question of the verb to claim.

When in doubt, I drop out optional bits that might be confusing things and see what I have. Since we know that "more than one seller" is singular, let's replace it with something simpler, such as "a hippopotamus":

There is a hippopotamus claims...

or

There is a hippopotamus claiming...

Now it's clear that because of the there is, it has to be claiming. You could, of course, restructure it thusly:

A hippopotamus claims...

or in your example:

More than one seller claims...

but if you want to use the there is, it has to be claiming