I would recommend that you try and talk to the guy selling the Monster XL, try and get the price down to 60 or 80$ . I've seen a Monster XL on my craigslist before and I regret not buying it.
Buying one on ebay is nuts, because the shipping cost of selling some 4 foot long heavy box is high. You are better off to drive there and take a look! If it really is in good condition it might be worth it. Much better than spending 100$ + 40$ shipping.
Of course a CPS 1500 is a much better gun for battle... but still. It's the biggest water gun ever produced, and I want one.