In the case of a picture of a product, the potential issue is trademark infringement (passing off). And sometimes the shape or some other aspect is also trademarked... e.g. both Fender and Gibson headstock shapes are trademarked (see Lawsuit Guitars); and the Gibson Les Paul, SG, and other body shapes are also trademarked (among many other things)... so it may not be only the brand logo that is of concern.

You never need a release to **sell an image as a photographer; it is the final use of the image that determines if a release is required, and it is the final user who needs the release... i.e. the user who buys it from the stock agency needs the release. Logistics dictate that it is normally the photographer who initially obtains the release (which must be transferable in this case).

If an image might infringe copyright/trademark, and there is no release that can be transferred with it, then the stock agency may list the image as being for editorial use only (which falls under the fair use/fair dealing exclusions); such as this example on ShutterStock.

Alamy presents it a little different by disabling commercial license sales, and changing "website" to "editorial website" license (personal use is always allowed). However; the "magazines & books" license could be a little erroneous... incidental use of one image in a magazine/book would be fine; but aggregate use in a photo book could be problematic.

In any case, it is not their responsibility to ensure the end user has the appropriate release for the final use... they don't actually have to tell you what you can/can't use it for, nor categorize/disable the sale (but it is good business).

**there are some situations/countries/etc where you may need a release/permission in order to initially take a picture, or to make the picture public if it is of an individual. But I am assuming a legally acquired image of a guitar as described.