Create a dummy code or two to handle these items, but realize it throws off your margin.
Example: Create an ILC (item lookup code) of ACC for miscellaneous accessories, or PART for miscellaneous parts. The description might be 'Enter a Description' or simply Accessory.
Mark the item as taxable (create another entry if you need a non-taxable item too). Leave the retail price and cost at zero, and check off 'Prompt for price at POS' under Options.
When this item is sold at the POS screen, arrow up to the description and overwrite 'Enter the Description' with the actual description of the item being sold.
We do NOT recommend entering a cost for these items since it will fluctuate wildly. If the items are not in the inventory database, technically they have no cost as far as RMS is concerned. Plus entering a cost will throw off inventory valuation.
For the same reasons you should NEVER receive these items into inventory.
We also suggest you mark the entry as 'Inactive' and 'Not to be placed on Purchase Orders' (Options tab under Item Properties). Inactive means it won't show up in normal searches or reports, but you can still enter the ILC in POS or run specific reports on these items.