What do you mean by "file"?
You mention "file" in your title but do not describe any in the actual post so I am being careful to consider that there may be more to your problem than is described there.
I would not use a separate table for purchases and sales. A single table can list all items with a status field that identifies whether or not an item is for sale, sold, or any other status that's useful. You can use finds, relationships and/or portal filters to work with just those items in each such category. Then, instead of copying data from one table to another, you just select a value from a value list in a field in that item's record.