The question wasn't migrated; the OP deleted it and reposted on stats. Here's your text to copy and paste there:
http://stats.stackexchange.com/ will probably help you better with this, and give a more comprehensive answer. I'm not a mathematician, but I suspect there are multiple ways to solve this issue.
As a programmer this is how I would tackle the problem. I'm not skilled enough to tell you if this is sound, but for simple data it should be acceptable.
Depending on the type of data, it might be acceptable to have cut off amounts. You will probably want a rolling average (often used in stock markets) that takes the average price over the last n months, this helps negate the impact of inflation, and then have a $n cuttoff or a percentage based cutoff, that is, any value that deviates +-20% or +-$n of the rolling average will be ignored.
This would work quite well for relatively stable markets, if your entity exists in a volatile market that fluctuates wildly then you probably want to find a different approach.
You also need to seriously consider cutting data off, you mention granny's yard sale which is arguably a legitimate cut off, but you need to accept that you will probably be losing legitimate data points as well that could have a significant effect on your results.
But again, there will be multiple ways to achieve this.