Hello Delphine,
It depends on how your tables are populated (manually, forms, automations...)
If you input the data manually, you may want to input the data in table B first (all information related to the new customer) and then add a "linked record" to table B in table A.
You will then have the possibilty to add lookup fields in table A to bring down information from table B (customer name, mail...)
Hence, each time a new sale is inserted in table A, you select the customer who bought the item and all information will be displayed.
Regards
Pascal