The 7 that recently sold on ebay were G551JW models. They all had i7-4720HQ processors, GTX 960m GPUs, and some had 16GB of RAM. All 7 sold for 475-700USD, depending on condition.
If yours is indeed in perfect condition, my best guess would be $600, give or take, due to the older GPU, as well as the marginally weaker CPU.