There is this zombie guy and it say he only serves zombies, is there a way to actually become a zombie? if so, what does he sell?

That was part of the love epidemic plot. You needed to be a high level zombie in order to buy from that shop. To see what he sells, you can go to SubetaLodge, Item Lists, and then select The Zombie Den from the Restock Location drop down menu, and then you'll have all the items that restock there.

You cannot become a zombie anymore, that was part of an event earlier this year ~ the Love Epidemic. However there's chance it might be able to become a zombie again next year, but it's not sure. C:

And for the Zombie Den ~ it sells zombie-related stuff like Minions, Books, some clothes...

oh, i see I love plots, but you get annoyed with yourself if you miss out on a really fun one!