Hey Cindy, glad to hear your store is a good one!
For the focus issue, I only realize you're using an SDM lens, I don't know about the 18-270 but my 50-135 isn't my favorite focus wise. The SDM does lag a bit and doesn't feel as direct as a screw drive one. I haven't read many reports of SDM issues with the 18-270 but maybe it's acting up. And SDM is pretty slow, noticeably slower on some lenses and the few ones I have do like to refocus once in a while.
Would it be possible for you to try your camera with another lens, preferably a screw-driven one? SDM might not be ideal for your shooting style. You can get a Tamron 28-300 on the cheap for Pentax K.
One thing to remember though is the minimum focussing distance of your lens is 49cm (~19"). This might be a problem, the lens will never be able to acquire focus on an object closer than 49cm. It is however the exact same minimum focus distance of the Tamron 28-300.
Just to be sure, when you half press your shutter, you do get a focus confirm Bip and the corresponding focus point lighting up in the VF? Because no Bip will mean that you don't have focus, if you move, it'll try to focus again. And as it was mentionned, no focus, no release in AF-S (or AF-A which no one should use). If you don't get a bip, you may be too close, or in the case of the picture above (maybe), your subject is moving faster than the lens can move or it might be too dark (the 18-270 needs a lot of light to focus, especially at the long end)
Originally posted by CindyV I mean I could buy a camera that costs a fraction of the price and not have to deal with this.
This is true but remember that what you are experiencing is not a typical problem, you've got most of us baffled with the issue. But as it was said before, every DSLR shutter button works the same way, every bridge camera and every P&S I tried too. And it should work as you want it to. Half press, AF focussing,
BIP, move while staying half-pressed maintaining focus and then fully depress to release. I tried with all my cameras (K5, K-r, K100D, Q, cheapo Panasonic P&S, the girlfriend T4i and Sony A57) and I cannot get them to act like your K-3
But if you're not happy with what you've got, might as well change. A K-3 is a pretty expensive piece of gear, especially if you don't like it. But try to eliminate possible issues with other things, like your lens before returning one of the best available APSC camera available at the moment...