It's not clear if you form design is correct for this. You ought to have a main form for the purchaser and/or sales info that is on the 'one' side of the transaction (this would be sales date, location, purchaser name for example) and a subform for the sales items as the many side. The link between the 2 is made via (usually) one field that is common between the 2. Sales number is probably a unique value (at least for a particular location) and would support this. The many side table (tblSalesDetail) would contain the PK (salesID) as a foreign key (FK). To achieve easier item selection as you mentioned, you can use a 'find as you type' feature for combos and enter one item at a time in said subform. You can also multi select from a listbox, but then you need code to append every selected item in the list into the sales details table.
If that's not making sense, research forms with subforms.
EDIT - there is also an 'Auto Expand' feature of combos. If the property (see property sheet for the combo(s)) is set to yes, the values that appear will match what was typed.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.