It is in a lot of cases. Usually people looking at PCs this old are looking to replace individual parts for repairs, for example, someone who already had a 6300 or 8320 but their motherboard died and they don't want to do a full upgrade yet. That's not to say you can't get $150-$200 or so from someone looking for a basic budget PC (assuming Windows is genuine), but that market is limited.
"Looking to expand your movie possibilities? Here is the perfect low cost system for that.
Low profile case that fits in your current entertainment center. Quiet, HDMI direct to your widescreen TV, blah de blah de blah..."
Yeah, if you can get those price, great. But most of those prices are not particularly competitive. In addition to the ones USAFRet mentioned, $50 may be too much for the power supply if that's a CX. If it's a gray-and-black lettered CX, you're treading on the edge of a cost for a new one with a full warranty. You can get a Silver-rated Vengeance or a gray-and-black lettered CX at those wattages around $60. If it's the older crappier one with the green letters, I wouldn't pay half of that for a new one.
What's the motherboard? You said MSI at the top, but Asus a couple posts ago.