Can I be sued for selling a product that is only "patent pending"?

Can an application be treated as an issued patent?

Section 154(a) describes the patent grant and its term beginning at issuance. A pending application is not that grant. Obtain the application number and check whether the sender is also relying on an issued patent; do not assume the label tells you the full portfolio.

Can earlier activity ever support a later royalty claim?

Section 154(d) permits a reasonable royalty for certain activity between application publication and issuance, subject to conditions including actual notice of the published application and substantial identity between the published and issued invention. The right depends on a patent issuing and satisfaction of the statute. Merely marking a product patent pending does not establish those conditions.

What should I do with a pending application notice?

Preserve the notice and published claims, identify the product at issue, and monitor the application if the potential patent matters to the business. Ask counsel to compare any later issued claims with the publication and product before concluding that pre-issuance royalties are available.