Yes, I do it all the time.

Some of my best deals came from private owners that were not collectors or hobbyists in the world of pinball. PinSide accounts are only relative regarding credibility. Some people do join just to sell something. Some operate businesses but have no interest in PinSide drama.

Many non-actives don't post for years, but have more experience than most on this website as soon as I recognize their real name.

Roger Sharpe has a PinSide account, is not active, and does not post.
Is he legitimate in knowledge and trust?
Yes.

Machines, parts, manuals, backglasses, etc.

Be vigilent, be dilligent always.
"If it walks like a duck, and talks like a duck, it is probably a duck."

If you have fears of the unknown, go find another savvy collector to help you validate. They don't need to be or even are always PinSide members.

Networks are strong, this hobby is still niche small in the grand scheme of world. Let them help you find what you want or need.

No need to travel the pinball road alone.