I want a seller to publish new posts on my site and in return, I pay him on Fiverr, all discussions would take place on fiverr and what he/she has to publish then based on that the seller can send me an offer to start the work. Would this be against the TOS?
It’s not against the spirit of the ToS, at least; if you need the seller’s mail to set them up so they can log into your site, however, to be sure, in the place of your seller, I’d explain this case to support and get express permission to avoid potential hassle.
If the seller explains why something that typically goes against ToS, like passing on an email address, is indeed needed to do a job, in my experience, it will be permitted, either for this specific job, or they may even get an okay for that gig for all such cases.
So, I don’t think it’s an issue, but for your seller to be safe, it probably would be better for them to get permission first if they haven’t already a gig that’s “pre-approved” for such things or asked before and got the go for future cases as well.
I think, it’s not against ToS. There are so many gigs offering this kind of services. The thing you can’t do is taking payment out of fiverr. Or using buyers email to communicate.