Often times a "bunch" of old trains will include rusty or "oxidized" track. It is seldom of any value and may be better discarded with one exception and that is switches which can be worth cleaning up by a collector. In all cases NEVER use steel wool to try to clean these rusty or oxidized items. Also many smaller train sets came with small transformers which are seldom worth the cost to ship them anywhere. Otherwise, the most value will be in the engines of the set followed by any accessories and then the cars themselves.
Do not discard any of the boxes or instructions that came with the trains. These are desired by collectors as well. Most of the engines and cars are identified by a number on the side of the car or engine which a collector can use to identify what you have. Pictures are most always needed to start to arrive at identifying and setting a value for the trains and a collector will most always request these.
Lastly, do not attempt to clean your trains with any abrasive, steel wool, or cleansing agent of any kind. Many of these will do more damage than good. A true collector will know what should be used and preserve the value of the trains.