Which word correctly indicates what is being sold?

The word that correctly indicates what is being sold is "Bicycles." This term signifies a plural noun, which points to multiple units of the item being offered for sale. In a commercial context, using the plural form suggests that there are several bicycles available for customers, thereby communicating clear information about inventory.

The other options do not appropriately represent the item for sale in the same way. "Bicycle" refers to a single unit, which may mislead customers into thinking only one is available. "Bicycling" is a gerund that denotes the action or activity associated with riding bicycles, not the bicycles themselves. "Biker" identifies a person involved in the activity of biking rather than the product being sold. Hence, "Bicycles" is the most accurate choice to indicate what is being offered in the context of a sale.