That's a 29r bike, so the mid-reach (47mm-59mm) brakes from your ebay auction won't cut the mustard. The best bet, being that you have the bike on-hand, is to measure the distance between the mounting hole and
straight down to the plane where the rims' braking surface is-- meaning, don't measure diagonal from the mounting hole and the actual brake surface, but the straight-line 90degree distance from the hole to that part of the rim.
I bet you'll need something more like these right here:
http://www.niagaracycle.com/categor...heel-front-79-99-mm-reach-oe-packaging-silver
http://www.niagaracycle.com/categor...wheel-rear-79-99-mm-reach-oe-packaging-silver
But, measure before ordering. Be sure; save $$.
Even if you'll be running smaller tires, the rims will still have the same 622mm diameter, so the reach from the hole to the rim will be the same, whether you're running 1" thick tires or 2.1" thick tires. And, being that the bike comes stock with 55mm tires**, fenders (+@10mm), another 5mm for the metal between the fenders and the mounting hole, and another @ 10mm for the drop from the rubber to the braking surface, that'd put you around 80mm, by my estimation. (**Assuming the specs listed on Walmart's site are correct
http://www.walmart.com/ip/29-Huffy-Millennial-Men-s-Cruiser-Bike-Grey/27831874 ) The thing is, if I'm off by a few mm
less, those 79-99mm brakes won't work. If I'm off by even 19mm
more, you're good to go with the 79-99mm jawns.
You're right to look for the standard-nut, non-recessed mounting set-up for your bike, and while the listing for those 79-99mm reach doesn't specify whether they're recessed or not, I'd be stunned if they were recessed. Regardless, it's possible to mount recessed-style brakes on non-recessed frames; drop me a line if you run into that problem. It's totally possible, and quite easy if you're not running fenders.
HTH
-Rob