I had one a few years ago and had it running for a little over a year. Overall I think its a quality tank. If the equipment is in good condition as well as the stand and glass I think 300$ is near impossible to beat in terms of value. I only had two real problem with the tank. One was routinely having to mess with the included skimmer to maximize it's performance. This isn't a problem for everyone, just seemed to be a common issue on the C-250 forums I routinely browsed. In addition to this, I much prefer the use of filter socks for polishing water which isn't really possible with the C-250. I was able to user filter floss but it wasn't quite what I wanted.
Ultimately, I prefer having a sump and the extended possibilities but if you're looking for a tank that can basically support any type of reef I think the C-250 is an excellent choice. Since it's been dry for awhile I'd ask for photos to check the condition of the tank. If someone cleaned it before putting it into storage I'd feel more comfortable since I'd assume someone that spent the time to do that obviously cared about the tank and had a motivation to set it up later. When you inspect it go over the equipment and make sure that the silicon is in good condition, look at the T5 reflectors and make sure both the lights and the built in hood fan work. Flip the tab on the hood and make sure the timer is still inside and looks functional. Be sure to check underneath the stand for Red Sea's included power bar and make sure all of the outlets work, switches too. Check the stand for any obvious water damage that will be a structural problem. I'd test the two pumps that come in the back of the tank as well as the skimmer pump to make sure everything is working well before you take delivery.
300$ is a great deal, but if the equipment is shot or a few pumps need replaced etc I would just go a different route.
