I think that you'll find that the #1 recommendation for a replacement notebook battery is to get it from Apple. There are just too many 3rd party manufacturers out there making replacement batteries to know which ones are ok & which aren't.
Also...if these non-Apple batteries are selling for 50%-75% of what Apple sells them for...they got to be cutting costs somehow. Cheaper quality battery cells...which lead to shorter runtimes & shorter overall life...and who knows about safety issues (which laptop batteries have had in the past).
Regarding the batteries you linked from e-Bay. If you're paying $38.44 for a replacement battery from them...and Apple sells them for $129.00...there's got to be something going on from a quality perspective. But hey...for $38.44...why not give it a try.
I would just pay very close attention (at least initially) to see if there are any charge/discharge issues...or if the battery gets unusually hot. And you might ant to see how much total runtime (hours & minutes) you get..to see how this $38.44 battery stacks up against an Apple original battery.
Just something to consider,
- Nick