|"Someone told me I could bring her IP address to my location"|
Well, your motives are irrelevant. Someone has been fibbing to you. It's quite impossible for you to use her IP address in this way.
Anyway, auction sites work according to username, not IP address (although they will obviously check IP addresses if they see suspicious activity). For example you could log in to somewhere like eBay from two completely separate computers using the same username (a lot of people do that so that they can use home and work computers for this purpose - or even just using a laptop at a different address).
Just log in with her details on your PC - no need for subterfuge, which is only going to arouse the suspicions of the auction site. Don't try to complicate matters when there is no need to.
You'll have to excuse my suspicious nature, but I used to work in computer security and I know all the tricks that people try.