as far as buying inside and outside mic's, I have found that if you are intent on buying used tool's then buy from a old machinist that is retiring and check calibration date's/stickers,, if you find the right person they will lot's of time's show you the in's and out's of proper useage and care for the tool's, most of the old fella's that I have bought from are usually pretty understanding and are glad to see the tool's that they have made a living with for year's are going to a good home and will be taken care of properly.