Of course, it is easier and simpler to sell anything via a third party, especially if they speak the local language and relieve you of the burden. However, that’s more about convenience, the real reason to use an agency, a good one that is, is to be sure your property gets the most international exposure and you achieve the maximum possible price.
No matter what sales yarn an agent spins, no one in the Bulgarian market can sell a property for more than its really worth, the most any agent can get for you is the market price. But, almost all agents operating in the international owner market will want to sell your property for less than its worth to make it easier for themselves. These you must avoid (see this section on scams)