Ive taken .2mm off a seat post before. Sanded down from 27.2 to 27.0 because I needed it at 27.0 for fit and also because there were some gouges originally. That isnt going to in any way negatively affect the seatpost- they are simply too thick for .1mm around to compromise safety. It did smooth out the gouges though.

As mentioned- taking a seatpost that fits a bike and then sanding it will make it smaller than what you need for the bike. I would instead buy a seatpost that fits and is in good condition, or is .2mm too big for the seattube right now and will fit once sanded.