I do this for fun, I have a full time job, I tend to only buy units that look interesting, that are big, full and clean. I donate the cheap stuff, give away stuff to friends & keep the really interesting stuff. The tax break helps & I try to sell enough to cover the cost of the unit.
I started buying 6 months ago and in that time:
Spent $6200 on 18 units (average of about $350/unit)
Sold ~$5800
I have kept/giving away to friends about 2k worth of stuff, have about 2k in inventory of stuff I need to sell, and donated god knows how much.
Seeing others peoples numbers are interesting though, using MovieMan's numbers of 1.8, to achieve, say, the median US average of 45k,
$45,000 profit / .8 = $56,250 would have to be spent on units.
You would have to sell the stuff from those units for over $100,000. Thats about $2000/week in sales every week of the year.
The number of units one would have to buy depends I guess. From what I have seen the average price where I live is close to what I pay, about $400/unit. Meaning one would have to buy 140 units a year.
Again, I do this as a hobby, I spend on average 5/hours a week messing around with this. But I often wonder how the people who do this for a living make a living, and, in my mind, the numbers don't add up.