are we talking about full height 5.25 drives ? or full height 3.5 drives ?
Cause it matters which you are referring to.
If its 5.25 then sure they have value if they are still operational, possibly quite a bit of value to the right collectors and even ones that are dead are still wanted for repair purposes. if its 3.5 drives then they also have value to the right people but only certain drive sizes are really worth anything. (10 - 500mb drives if they still work can be worth a nice amount, 500 - 5gb also sell well for people wanting period correct builds)
But generally most collectors will go with SSD, CF or SD>CF setups as they will be faster and more reliable.
Powering them on? I wouldn't bother with anything more than a basic DOS on floppy disk setup with a few basic tools to copy the drive contents .. like Laplink or other serial setup, best to keep it as simple as you can for quick dirty HDD backups as you may not have a lot of time with old HDDs that have not been used in a number of years.
Also make 100% sure that the PSU you use provides stable voltages to the PC before you use it just to give yourself the best possible setup.
Edit - If its old MFM/RLL drives then they may be best left unpowered unless you have good knowledge of how they function as they can require a little help to get the actuators moving again if they have been sitting for some time.
Oh noes, the cap let the shmooo out 😁