Some great answers here already, and in general I agree with the strategy of focusing on just one domain where possible.

So, if you agree too, the next question is how do you come up with a new name/brand/domain to capture it all?

I've done several branding/naming projects, and my recommendation is the following:

1. Nail your positioning. This means, in very few words, how are you positioned with your target customer, and against your competition? It's not easy to simplify this down to a couple sentences, but it is imperative.
2. Find a name that speaks to, and supports the position you are taking. There must be a strong tie between the position and the name, otherwise creating messages (marketing) will be very difficult.
3. Find a domain to call home. The beauty of a highly distinctive position and related name is that finding an available domain becomes MUCH easier.

Let me know if you'd like to talk more about the positioning, this is the hardest part and good to get help on!

I'm a positioning and messaging expert, working now and previously for several software companies in CMO type roles.

Ideally you'll want to focus on your propositions to both sellers and buyers independently. You concept is dependant on providing the mechanism for bringing these markets together, so a failure on just one side means a failure for the system as a whole.

Focus early on what works and what doesn't for each group, and what's broken in the relationship options they have today.

Nail your positioning and messaging to each specific side of the relationship, then create test campaigns and landing pages for each.

I wouldn't try wrap it up in one landing page just yet, make sure you understand what's working for both the buyers and the sellers independently first.

Good luck, drop me a line if you'd like to speak further on it!