If coins and stamps are worth enough - and many are then you need to start treating your assets with respect. I would consider things like markings that show up under ultraviolet light or a microdot - things that don't affect the article. Get the marking(s) asserted in some way. Make the grader aware that you have taken steps. If the value is great enough, then you might pay out for a minder that follows the article throughout the process.
Perhaps there is a market for an intermediary service of some sort to do this job. Anyway, this has all been done before. You need to do a risk assessment on your stuff and protect yourself in proportion to the potential value of your things. Getting valuations done for any valuable thing is potentially problematic but I'm sure if you have a chat with, say, a jeweller they'll tell you how to get the job done safely.
Sure enough, I went to a coin dealer and that's exactly what they did.
You're paying for the grading/encapsulation as a service, independent from any future sale plans, so they have no stake in under or over-grading any individual coin. They got their service charge (typically $15-100 or so depending on estimated value and turnaround speed) and they'd lose community trust if they were consistently too lenient.
The market only trusts a handful of firms, most of who have been around since the 1980s or 1990s and certified millions of coins. Most other brands are "price it as you would an uncertified coin, caveat emptor" or perhaps even worse, as some sketchy dealers would set up a fourth rate grading service as a means to make low-quality inventory more appealing.
Heck, you can't even hand a collection to a grader. He'll swap out your good coins for lesser ones when you aren't looking.
Same with stamps.