Red, "merchandise" isn't a verb that is arrived at by adding a suffix to a different root, so -ize is incorrect.

You could further distinguish words by their origin, e.g. if it's of French origin you could use -ise (in British English only) and -ize if it's of Greek etymology etc., but in being consistant, -ize should be used for all these verbal forms, and the nouns that are made form them: realize, realization; organize, organization. These can be spelled with -ise in British English, but worldwide -ize is used.

According to the OED the correct suffix is -ize, because of the Greek root it comes from. -ise is a more modern British spelling, but originally both sides of the Atlantic used to spell it with a 'z'.

"Analyze" however is incorrect. It should always be spelt "analyse" because it's not derived from the same root.