First remove any debris with compressed air. Then we use a new microfiber cloth - and if needed - a touch of water on the cloth (very lightly damp) Water is always my first choice. Don't use windex! The ammonia will damage the display coating. More stubborn sticky stuff can be hard to get off, so we then use an ammonia-free, water based glass cleaner applied to the micro fiber cloth. Dry it quickly, and don't swirl the cloth - use straight, even strokes in one direction. LCDs are fragile, so it is best to lay them on a flat surface when applying pressure.
You can use the optical drive sensor from a 2009 and newer iMac to send temp info on the 2010 and 2011 logic boards. Just compare the HD sensor jack on the logic board to the optical drive sensor jack and you will find the right one. The sensors are cheap, usually about $20 and will report skin temp on the drive cabinet adequately enough to keep the internals cool without over-revving the fan
In Florida retail resellers must have a second-hand dealer's license. It's kind of like running a pawn shop where you are required to take ID from the seller, log the serial numbers into the FL Dept of Law Enforcement data base, and there is a waiting period before you can sell. Obtaining a 2nd hand dealers lic. requires a background check.