I Lifted this information from another website,i claim no credit for the content but i feel the information is very useful.

In the market for a stove, microwave, or refrigerator? If you are considering purchasing one used, you may want to think twice!
Used appliances such as stoves, microwaves, refrigerators, dish washers, toaster ovens, etc are notorious for being infested with German Roaches. Out of all of the above listed appliances, I would have to say that refrigerators are the worst.
The ideal living space for a German Roach is one that is dark, warm, has food, and water. A refrigerator provides all of these elements. When people think of refrigerators, they most often associate them with cold temperatures, rather than warmth. They often overlook the heat that the motor is putting off. The heat from the motor, the darkness of the underside of the fridge where the motor is located, the food crumbs that fall all around and accumulate around the fridge, and the condensation from the ice maker / water line provide German Roaches, and other insects, an environment that they can not only survive in, but absolutely thrive in. Dish washers offer all of the same tempting conducive conditions as refrigerators.
Stoves, microwaves, and toaster ovens provide roaches and other insects all of the above listed conditions, with the exception of water.
Televisions, stereos, computers, etc all have dark spaces, and give off plenty of heat, thus making them great living spaces for roaches.
For the little bit of money that you might save by buying used, I would have to say that it is absolutely not worth it. You could be creating an absolute nightmare for yourself, and could end up spending much more money to get rid of your pests than it would have cost to buy a new appliance.
How bad can it really be?
I have seen appliances that have just a few roaches in them, all the way up to what must have been several hundred in one refrigerator. One of my customers had so many roaches in her appliances that they caused the dishwasher to completely stop working, and caused a fire that destroyed her oven. Keep in mind that, even if you can’t see any roaches running around, a pregnant female could have laid her eggs inside of the appliance. A single egg capsule can contain dozens of roach eggs.

Whilst i don’t completely agree with the author i think if you thoroughly inspect any used appliance you can usually see and signs of infestation, the signs of a roach infestation are really obvious.

Think about where you are buying from? if buying a used appliance privately then it is easy to gauge the kind of environment it has been in, if the place is dirty and very untidy or if you see a kitchen with food debris then the appliance may well have unwanted guests.

With used appliance dealers the only real way is to look, don,t be afraid to ask where the dealer obtains his inventory and if or how he ensures his units are pest free.

Lastly don’t think newer appliances are roach free, we recently purchased a whole set of near new kitchen appliances from an empty property, these units were less than 6 months old and guess what? yep they had roaches.