Online business domains design plan

Hi,
I'm planning to start a quite serious online business and I need you thoughts on domain and seo related subjects.

Background:
It will be a some kind of a marketplace. I will help the buyers to find suitable seller, help sellers drive buyers to their offers etc.
So there are two kind of my clients: sellers and buyers. And there are a few services: sellers search for the buyers, membership forum for the sellers to exchange knowledge, sellers mini sites (I will host them), some general knowledgebase.
I got also one nice domain for this business (let's call it maindomain.com ).

Question:
What is better from the users (usability), business brand and seo (site authority building, promoting) perspective:
- to create a subdomain for each type of service (eg. findseller.maindomain.com, forum.maindomain.com, seller1.maindomain.com, seller2.maindomain.com...)
- create separate domains for each service (eg. findseller4you.com , sellersforum.com, seller1.hostsellers.com. seller2.hostsellers.com)
- mixing it, or someting completely different?

Do you know a business with a similar model and how do their domains look like?

Looking for your serious advice!

Take care,
D.

Why do you even need separate domains/subdomains for each service, if they're going to be hosted under the same main domain and same design. Unless the services offered and design is completely different for each service, you can probably get away with just using subdomains and directories.

For example, for sellers, you can use seller1.maindomain.com but also allow them to use maindomain.com/seller1. For the forum you can do the same, maindomain.com/forum or forums.maindomain.com (but a subdomain is probably better for forums).

You probably don't need seperate subdomains or domains for your services if your layout/design is good, but you can always just 301 your subdomains or domains to a path on maindomain.com if need be.

For the users, the less they have to remember the better. IMO, having separate domains sort of forces you to create a new identity for each.

Thank you for your thoughts on this. You're right that overcomplicating is bad.
I'm considering different setups because each service will be based on different technology (forum, shop, wordpress mu, wordpress frontend).
I'm also aware that service for buyers and for sellers needs a different way of promoting (eg. different keywords to rank).

I also wonder if there's a difference in ranking a subdomain and a domain in google.

D.