Increasing the cost of the house works against the agents too, because of the banks. There are two gates that have to be passed through: the buyers must be pre-approved for a mortgage that is sufficient to pay for the house, and the bank will send an inspector to the house to value it before approving the final mortgage.
A seller won't accept an offer from a buyer that doesn't have evidence that they can afford what they're offering. The buyer's agent wants the offer to be accepted, so they won't let the buyer offer more than they're pre-approved for (and often somewhat less, to reduce risks that the deal will break down before closing.)
Once the offer is accepted, the bank inspection has to happen, and if the bank determines that the house is not worth as much as the buyer offered, the deal will fall through and neither agent gets paid. So, both agents have an incentive to keep the cost below what they think the bank valuation will be.
So it's really the bank that's in control of property values, not the agents. In a housing bubble that's a bad thing, because the bank is lenient in both pre-approvals and valuations, and agents incentives are to follow the bank upwards. But in the current climate when the banks are cautious and risk-averse I think the incentives are working well, because the banks and agents are countering each other.
As far as what the agents tell the buyer and seller about the price:
- If the house is sitting on the market and no one is making any offers, the price is too high. So yes, the sellers agent will tell the seller to reduce the price, because they want the seller to sell. (Getting the house to sell is the agent's job, after all.) Just sitting and waiting at the same price is bad for the seller, because it makes other agents and buyers think there's something wrong with the house if it has been sitting on the market for a long time with no offers.
- If a house is priced correctly, multiple buyers will want to buy it, and they'll all make offers. The buyer's agent knows that the seller will choose one of the higher offers (though price isn't the only factor), and they want their buyer's offer to be accepted (that's their job), so yes, they will tell the buyer to offer more. My wife will tell her buyers how many other buyers they're competing with that she knows about (agents can't discuss details, but can give vague impressions to help each other out), and that they should offer more than the list price but not so much that they would regret the amount afterwards. In other words "Offer what this house is worth to you, so that you won't wish you had offered more if you don't get it, and that you won't wish you had spent less if you do get it."