Got ebay shipping tracking number never got item>!>>!?

i sold an iPhone 4 and an iPhone 3GS from ebay and both of the sellers paid me. I sent them both a tracking number as proof of delivery but they said they never recieved the item and it's been 2 weeks since i sent it out. what happens if you shipped it to the wrong address?? they told me they want a refund.. what do i do? am i the one who has to give them a refund or is that on ebay? cause if i have to give them their money back i'm out of my items which was worth alot as well! cause that's ALOT of money and money i don't even have. and it's not my fault that they never got it cause i gave them a tracking number. please help!!