I use Monoprice for a lot of stuffs, but for the 12AWG wire you are asking about I was able to get a 250 ft roll of CL2 wire from Cmple for 60 USD through their EBay store, shipping included ... its more on their website

. It looks like exactly the same wire Monoprice sells as well.
It's pretty close, plus it's CL rated which means it's safe for in wall use. The only issues that I can find are that the ebay wire doesn't say oxygen free and it's from an ebay store.
Frankly an ebay store can say whatever they want about what they're selling. With monoprice you get a few things you don't get from an ebay store.
Certain guarantees, like product quality and that fact that their products meet the specs that they're advertising (like CL rating).
Buyer protection from issues, such as not getting all 250', wrong product, wire gauge being mislabeled etc. Monoprice has an excellent reputation for making good on mistakes and defective products
You also get a lot of support for setup help and installation.
There's also a pretty good return policy.
Now I'm not saying the product you bought from the ebay store isn't what they claim it to be and judging by the feedback it seems like they really deliver. However there is also a lot more risk involved.
How many people actually checked wire gauge and not just the number on the jacket? How many people verified that the CL rating was the real deal? How many people unrolled the spool and checked to make sure they got all XXX'?
Sure there's ebay buyer protection but that's usually a lengthy process and a huge pain in the butt. You have to cover your own butt and provide evidence and proof of correspondence etc etc etc.
Running an ebay store lowers overhead cost significantly compared to a B&M store or even an ID store, so it's more than possible that their products are the real deal and they're just cutting off more fat to sell products at even lower prices and as long as you're ok with the risks involved and you're done all you can to minimize your risk then that's a great deal
