Need to reload OEM OS on 7-8 machines

I have 7-8 older Dell Pentium II and Pentium III desktop systems that my employer got rid of when they went out of business. I'd now like to reload the OS on each of them and sell them. Using the Dell service tag on each machine, do I need to order the OEM replacement OS disks from Dell for each individual machine? I don't really want to pay the $20-$30/ disk or whatever Dell charges just to have them send out multiple copies of the same thing. Couldn't I just load the OS on each machine with the same Windows disk and then register the OS with each unique MS Product key that Dell has for each PC?

I see others who are reselling multiple older PC's complete with a freshly installed OS on Craigslist all the time (Dell and otherwise)--How are they handling the OS issue? They can't be paying for a licenced copy of the OS for each machine themselves (at least I'm assuming they're not--not for a computer that's only worth $100-$150). I guess I'm not all that clear on how OEM licences work. Surely there have to be a lot of people wanting to reload OEM OS without the original OEM disks. Just not sure of the easiest way to do it.

Some clarity on how to reload the OEM OS in the easiest way would be appreciated