I like the idea of modeling duration between sales. One distinction I might need to make clearer is that I am not attempting to predict how many sales are likely to happen - but rather weather or not another sale is likely to happen in some reasonable time frame (to use in deciding if another unit should be purchased preemptively). For items that sell a lot, this is always 100% -- but for new items, two sales in the same month may indicate something very different.

I'll look into Reliability more -- but I'm not sure why you think it won't work. Imagine it's June, and we have sold 1000 rubber bands. Should I order another rubber band? Probably - I'm just wondering how I might calculate how sure I will be.