Yep Mike, Jim[moe] is correct here. Your first concern would be that this rim is a 36-spoker and holds the same number of spokes as the stock BA hub does, and which would make lacing this 21in rim to the stock BA hub and then mating it to your existing forks and brake caliper a breeze.
However, after counting the number of spokes on the wheel you've just purchased on ebay, unfortunately it appears that one is a 40-spoke setup(unless I've miscounted, that is), and thus while this won't make it impossible to mount this one, you'll be into a lot more work in order to make it work.
This means you'll have to(as Ryk noted above) find or machine a new axle, and if you'll want a brake mounted to it also(and I DEFINITELY advise that you run a front brake, as over 70% of your braking power is by use of a front brake), the rotor would somehow have to be added to the hub.
