The Ross-Tech cables come with the software license and work with the newest software versions. The ebay cables usually don't come with a software license ($150 or so) and only work with older software versions (up to 409, current version is 607, 511 or newer is required for some interfaces like the phone interface). Cheap is never inexpensive and if you plan to have another Audi anytime sooner or later, get the real Ross-Tech cable as Hex-CAN version, with lifetime updates and great support. Or if you need CAN only (A3, Golf MK V, TT MK2) get the Ross-tech Micro Interface).
I was going to buy the cable off ebay but asked the question on NAV PLUS before i did and i am so glad i asked as now I have bought the ross tec cable which i was advised to do and i have to say i am so pleased i did i had a problem with my MFSW install and Ross tec where brilliant they helped me through it and helped me solve the problem.
you will not get this support from the ebay cable and you will not be able to use the full features of VAG com