Okay, I'll take a stab at it (figuratively)...
The spination is reminiscent of Melo zehntneri, although that plus the color of the spines makes me think if M. deinacanthus could be a better candidate. See if this photo seems to be a match: http://www.cactiguide.com/graphics/m_de ... _a_600.jpg
Edit -- spines look too small for a deinacanthus, so maybe it is a zehntneri.
Melos deserve their reputation for being difficult, since they're rot-prone and won't survive much below about 5C. To make it even more interesting, they also don't hold up well under extended periods without water. For whatever it's worth, two pieces of advice:Critical boo-boo edit
-- I posted this as -5C last night. I just corrected it above, and that is +5C
(= 40 degrees F). Sorry about that!
1. Since it appears that your Melo was sunburned around the cephalium before it came to you, I'd be concerned about letting rot get into the damaged area. If possible, keep the rain off, and water the soil without spraying a garden hose.
2. Getting your Melo through the winter is a challenge, although by no means impossible if you're careful. The trick is to give it some light
watering once a month over fall and winter. The soil needs to dry out quickly, otherwise you'll end up with soggy soil and a dead plant. The other side of this equation is to keep overnight temps at or above 15C while the soil is retaining moisture. I do this with my M. matanzus when I bring it indoors. If you can do the same, a week should be enough to ensure that the soil has dried out well before it's safe to put the plant outside before its next fall/winter watering. However, if your overnight temps are dipping below 5, it should be staying in anyway.
That bigbox store didn't do you or your Melo any favors, but definitely worth the effort to rescue it. If it doesn't survive, just remember that even more experienced growers can have trouble with them. But if it lives to see another growing season, you can be proud of your accomplishment. Please keep us apprised of further updates if you'd like -- and it would be nice to see photos of a positive outcome in the next growing season.
Best of luck!