Yes, there is always that possibility.
But there are a few things you could do to minimize your risk.

I would gently pull back the boot around the door and look in all the crevices inside, ahead of and behind for any kind of black or pink looking deposits.

Put your head in the machine and see if it has any off odors. If you gag, you know it's not a good machine.

also never buy a frontloader **without** internal heating element.
Mold grows up because of lack of real hot washes (140°F at least, 120 is not enough).

Also make some other questions to the seller about his/her laundry habits . Stay away from people that wash everything in cold : you can be sure they'd sell you a moldy washer

You can play dumb and ask the potential seller a bunch of trick questions:

You: "What type of detergent do you recommend for use in your washer?"
Seller: "Oh, you can use any detergent - we just use the cheap liquid stuff."

You: "Okay. And which cycle do you recommend?"
Seller: "We like to always leave it on Normal Cold/Cold."

You: "My laundry room is in a tight space. Is it okay if I leave the door and dispenser shut all the time?"
Seller: "Yes - we do, too."

You can probably think of more questions...

If you encounter these types of answers - don't get their washer! Liquid and/or non-HE detergent, cold washes, not allowing the washer to dry out etc. will probably result in mold.

HTH, Alex