Stores may not because very few items on the shelves are suitable in coastal salt air. Fasteners may be one of the easier items to assess. Not all grades of Stainless Steel are equal.
Note: 316L Stainless Steel Is commonly used in marine environments, but Marine grade has no standard definition, and a variety of mis-interpretations if we are referring to stainless steel.
A second point to consider is the relative corrosiveness of the environment where the product is going to be used.
Is it reasonable for the average hardware store to label or advise accordingly?
@Dennis44 has raised a very important issue by his example.
For those of us exposed to coastal or more extreme environments finding reliable products is a genuine concern. The majority of everyday products we purchase are produced to suit the average consumer’s needs. Many manufacturer’s do not produce products that are suitable for the more corrosive coastal environments. ‘Buyer be ware’ if there is anything in the fine print of the suppliers warranty or sales contract for custom orders. For custom made external products we might all expect the supplier would consider the location and environment in supplying a quality long lasting product. Although consumers are susceptible to style over substance and suppliers to offering the lowest quality they think they can get away with In a price sensitive market.
It’s a great topic and perhaps an opportunity for a buyers guide on, ‘What to look out for when buying fittings and products if you live near the sea or in a humid/tropical environment’.
The commercial and professional designers fall back to Australian Standards and specifying minimum requirements, for the expected environmental conditions.
For the average home owner there may be two hurdles. Firstly knowing what is the minimum product quality, or life condition to ask for. Secondly finding a suitable product or supplier who can confidently meet their needs. Not everything comes in high grade 316L stainless Steel, or if it can be done the cost of a one off special may be too great to contemplate. The risk for consumers is relying solely on a verbal assurance the supplier has heard your need correctly. A reality that the only product the supplier can offer you may be a poor choice but will sneak past warranty. Sale or no sale?