What bike is this? Need to find derailleur hanger
It looks like you need this one (Wheels Mfg 21, which they list for Giant Boulders of all years).

If you took the bike to your bike shop, they'd compare the derailleur hanger visually to other ones used by Giant and see there is nothing else like it. Or pull up a chart from Wheels Mfg and look up what bike it is and which ones could fit. 

So it looks like the one you have found on eBay is the right one.