For most of my guns, I've allocated one or two certain guns to individuals for a particular reason such as that person taught me to shoot, or introduced me to the SKS or the 1911, etc.
For some of my guns, I've allocated them to people that don't already have a gun and am gifting them a gun - sorta my own way to get them hooked or at the minimum help them defend themself. I am considering a caveat that if they sell the gun in their lifetime the profits return to my estate, to ensure that they keep the gun.
My goal is to distribute my guns to as many people as possible that I think will appreciate them, and also to give them each a keepsake to remember me, rather than just some cash.
I've never been the recipient of a weapon from anybody but my dad, but I'd have to say if you think that there's even a chance the weapon would be sold, is that really a person you want to gift a gun to? And do they know they are going to receive them; do they want a gun?
The earlier suggestions made about gifting them before you part sound ideal to me. Sit them down, one by one, explain what they'll be receiving, why you picked that gun (or those guns,) why it's important to you. Even if you hold on to it and just maintain an itemized list of who gets what, with their contact info, when you do pass.
Just the act of taking the time to talk to them about it is probably enough assurance that it will mean enough to them they won't discard of it to fund some other hobby as soon as you're not looking, I think.
I also like the idea of the handwritten note for the gun, which got me thinking about the Swiss K31's and how they had those custody notes under the butt plates. Not feasible for all guns, of course, but a gun history buff would really appreciate an homage like that, I think... I know I would, anyways.
And of course, I'll offer myself up for adoption to any of you guys who don't know what to do with your weapons... I'm a gun fanatic with only a gun or two to call my own, so far...