When we made an offer on a house, our (buyer's) agent had us sign a seller's agreement. What advantage/disadavantage for him and us?

Each state has different forms.
You should not be signing any form that has not been explained (although I suspect that you are OK).
What is the exact title of the form?
Is this a house that wasn't represented by any brokerage?

Hello j:

ALL Realtors represent the seller by default. If you wanted your agent to represent YOU, he needed authorization to do so. It means that YOU pay him, rather than him collecting a piece of the sellers commission.

excon

PS> (edited) I would have called the agreement a BUYERS agreement, though. I don't know why he called it a sellers agreement... Uhhhh, what did it say?

In my state there is no default; all buyers must sign one of 3 forms:
Exclusive Right to Represent Buyer (not used much), Exclusive Agency Right etc, and Open Right etc. Then if the house being shown is also listed with said broker, there are Dual Agency forms that must be signed.

I currently have 3 agents acting as a Buyers Agent for me, each looking for a specific items for me: A-raw property, B-foreclosures and C- particular type of home. In none of the contracts am I indebted to pay them at a purchase UNLESS, they find the property and I then buy it through someone else.

I currently have 3 agents acting as a Buyers Agent for meHello ma:

You think you do, but they represent the seller.. That doesn't mean they won't work hard to help you find a property. It just means they'll be paid by the seller - who is their client.

excon

A agent is not a buyers agent, unless they and you sign a specific buyers agent agreement. Payment by this varies, and the rules vary by state ( in the US)

I was only addressing your comment that "YOU will pay commission". Brian