Pallet wood doesn't usually last very long
I use whatever skips and Freecycle provide.
As long as it's fairly new,a coat of wood preserver and a couple of coats of linseed oil(then an annual top up coat)keeps everything waterproof.
Pallets are often made of the gnarly stuff that gets left over once the prime cuts have been taken but you can get lucky when that's all used up and they have to use the better grades.
You just have to prepare it for the particular task
I ended up with some lovely cedar when I made my Honey dryer.
I'd avoid using wood where it contacts the ground directly-I make my stands with metal legs
If you've already made them with timber you could put plastic feet or similar on the ends.
It's commendable that you are making use of material that would otherwise end up polluting.