The only problem with buying a broken device on ebay is you are not 100% sure of the condition of the phone's parts. Find a listing that has the right price, especially a but it now, and contact the seller to find out the condition of the phone, including the slider, in case you need to swap out the slider mechanism too (which sounds like the case).

I have heard of people managing to order just one spare part from that one site. Maybe, if you need a new slider, you can order one of each. I haven't personally tried to order from them myself. Still can't believe AT&T charges $340 to repair it.