Not necessarily, in fact, the opposite usually applies. When you're selling without an agent, you have the competitive advantage coming into the market. Selling yourself means you aren't paying an agent a hefty sum in commission as well as large amounts in marketing fees which ultimately means there's more room for you to negotiate on your price.
When there's more room to negotiate, there's more chance of agreeing upon a price that both buyer and seller are happy with. For example, even if you were to market and sell your property $5,000 lower than the market average, you're still saving money. You also know that you have your own best interest at heart and that you know your own property - agents will often push for a lower sale price to get the sale finalised and their commission in their pocket.