To sell or not to sell

In short, we're having a yard sale soon. The hardest part, for me at least, is deciding what is sold and what is kept. My over-analytical side can't do anything without laying out some rules first. So far, I've rounded up a good deal of things using the following logic:

if item will not be used in the next 12 months

  and item holds no sentimental value

  and item can be sold  /*meaning someone might actually want it (sorry, old underwear)*/

  then item gets sold;

else item gets kept;

So, this reasoning works pretty well for most things. But every now and then you run into an item that's iffy. Such as, a sleeping bag. Will it be used in the next 12 months? Most likely not. Does it hold sentimental value? No. Can it be sold? Sure, it's practically "like new". But should I keep it instead? Maybe. I'd hate to run into a situation 2 years from now where I need a sleeping bag and think, Crap! If only I'd kept that perfectly good, barely-been-used one we already had instead of selling it for 25 cents. 

Yeah. The verdict is still out, but I'm hoping I can remain steadfast and get that quarter. And more importantly, the extra space.