Hi penn919,
I am sure a lot of us here have an old 3.5" HDD laying around as do i,
firstly your question is not specific enough , there are more than 1 type of 3.5" HDD
Is it a sata drive or an old pata( IDE ) drive both are 3.5"
I understand you are trying to save money , but you are better off saving for a new drive
The reason i say this is that there are no guarantees as to how long an old HDD will last , it may have
been working for years and nearly at is lifes end when you buy it , or it may work fine and die in a month etc etc, you have no way of
knowing how much work the old drive has done and how old it is when you get it.
For the capacity HDD you want they are cheap to buy these days , don't get used, get new because you will get a warranty with it
and if it arrives DOA (Dead On Arrival ) you can get it exchanged for a new one
Also you will pay much less at an online pc parts outlet than what you will on ebay and you wont have to deal with sellers or get ripped off
and they usually get your stuff to you within a few days.
Save a few dollars a week and i'm sure you will have your drive really soon