The 'rule of thumb' that I go by is 10% of the retail price, if the item is in nice, used condition. If it is something that is never used, then maybe up to 50% of the retail price. Some items may not go for as much as you think they should, because they may not be as common to use as you think they are, so go into selling stuff with an open mind about lowering the price (if someone makes an offer) to get rid of it. If you are selling a top of the line, top dollar something, then don't expect to get as much (percentage wise) for the item; people are at a garage sale to get a bargain. If you are planning to keep the item if it doesn't sell, then by all means, leave the price a little higher, and you just may get lucky and sell it. Take Care!