The "market," made up of willing sellers and buyers, determines the final price, and that is the only "right" dollar value. Real estate agents don't determine listing or selling prices, they only advise their principals about pricing and pricing strategy. Some sellers prefer to price low and get multiple offers, some prefer to price high and expect lower offers. The best pricing strategy varies with the local area and changes over time. Comps for residential appraisals and CMA's are usually based on recent sales, not current listing prices. It is unlikely the agent you are referring to is having any material effect on market prices, but rather the forces of supply and demand are affecting your market.
To answer your question there is no practical way for the average person to stop this from happening.
If you were a savvy investor and you thought prices were too low, you would be buying up the inventory and reselling it for a profit.
When you are ready to sell your own property, seek the advice of a local professional, and list at whatever price you choose. Satisfy yourself that the price you ultimately sell for is the right price.
Best of luck!