I use BBQ skewers and toothpicks to stir paints and apply glues; I get them at the dollar stores. At about one cent each they are cheap enough to use once and throw away. I also cut the skewers so I can use the same one as many as three times to apply glue. Simple Green is a household cleaner that will strip paint easily. It is also non-toxic. Buy at Wal-Mart or Sam's Club in a large quantity. Don't pour it out, save the cleaner to use again and again. It will strip the paint off a large number of models, even though the Simple Green will turn black after the first couple kits.
I almost never throw anything away, and generally don't sell hobby items. I will trade them or give them away, particularly to kids. Those kits that have multiple versions, two turrets, or extra wheels, can be used for conversions or added details on other kits. Buy quality paint brushes and take good care of them. I have had some last as much as 20 years. Store them upright. I made a paint brush stand out of a 2 x 2 piece of wood by drilling holes in it, about 30 years ago! Clean the brushes and then dry them with a tissue. Never store them dirty, clean them right away. Also use a tissue to clean the rim of your paint jars, a tight seal will keep your paint fresh longer, sometimes ten years or more.