I guess this would work as long as not everyone is doing the same
And like you said you are probably loosing a bit of money, since some people are always cutting prices for whatever reason they have. For example I am thinking ores which nowadays are usually priced high because of the strong demand to level various professions (I just leveled engineering and blacksmithing which reminded me the amount of copper / tin you need is just huge) while the supply is limited I suppose because of less lower level content farming. I was leveling some additional alts to prepare for WotLK and selling the extra stuff, and everytime I checked the AH there were some items listed for maybe half the price compared to the majority. If you notice that you quickly discard those and get to the actual lower end of the price brackets, but like I said it depends on how much attention you want to put in.
I guess it all depends on your local realm economy and market stability at that point. Various addons will work more or less accurately and yield different results I am sure. I play on an old, well establish realm where the economy is very dynamic, and Auctioneer works great. I also play on a recent low population realm and it is mostly useless because prices greatly depend on whatever the daily offer is.