1990 Lesabre

I have a few question and would love some advice, input, anything.
I have a loud knock under the hood of my lesabre. We are pretty sure it is the harmonic balancer. Even with the belt on you can wiggle it and it makes the knocking noise.
1) Is that the proper way to diagnose that problem?

At advanced auto a new part is $95 + tax. I found a new one on Ebay for $58 the seller has great feedback but has a no return policy unless they ship wrong or damaged part. I have never bought parts from ebay so I'm not sure it's a good idea.
2)Your opinions on this?

I've heard that you have to have a special tool to change the harmonic balancer.
3) Can you tell me the tools that I will need and if this is something that can be done DIY?

4) If this could be anything else besides the harmonic balancer please give me some ideas.
Thanks a bunch for any and all input. :)

I think you found the trouble those balancer are known to go bad .the rubber rotts out remove the tire and belt to get a better look at it it will also give you acces to remove it. im not sure but i think that one just slide on so no puller is needed to remove it