From the site down below:
"Not for resale" is commonly attached to items that are given away in promotions or that are bundled as part of a special offer. For example, when a display of hand lotion is set up in a store, there might be a sample on the counter for everyone to try. This sample is likely provided by the manufacturer. It will be marked "not for resale" so that the retailer doesn't sell it as part of its inventory. The manufacturer wants the sample to be available to the public at no cost to the retailer in hopes of increasing sales."