Dates: length of time and averages

On a spreadsheet I have one column where I enter the date I bought a property. Another column where I sold the property. A third column that shows the length of time in between where I held the property. Right now, as I have it, if there is not a sold date an error message shows up in the Time Held column. It would be nice to be rid of that. If there is neither a date bought nor date sold entered yet it registers a 0 in the time held column and it would be nice to be rid of that. Just to keep things clean but not essential. How can I do that?

Finally in the footer of the time held column I would like it to show what is the average time held among all the rows. How do I do that?