don't know where McKinney Texas is in relation to one of the FEMA lots, but.....FEMA is selling used Katrina Relief trailers (not pop ups, travel trailers). i would never by one from just their auction site, but other posters have confirmed that if you are near one of their lots, you can go there and view them and that some of them are actually in pretty good shape. (some are not, not buy a long shot)
but - if you are specifially looking for a used pop-up, try any RV/camper sales lots in your area. be sure to stick your nose in them and give them a sniff before buying. any musty smell and keep moving. mildew = very uncool in a pop-up. in my area, you can find used campers at the end of folks driveways this time of year and in the fall. My inlaws found a great one for 2K that way a few years back. We were looking at lots and lucked into a new end of sales year Viking basic 9 foot pop up for 3k. it has been worth every penny.