I have the new ones (brown SK LX), they are indeed thinner, with a bigger screen and that i do like... i used to have the SK3 and it's great, it's pretty much all the same
it's really a great phone, but it's true they don't sell them anymore at the t-mobile stores because the new Sk LX and the SK Slide is out now.
good luck with your sale, it is truly a great phone