Learn English – Can “as well as” be used in “A, B as well as C” structure

"Get well" is appropriate. To me, however, the phrase is something that would be written, for example in a "get well" card. It is a little too cliche for me to say in an everyday conversation.

On the other hand, "Be well" is usually an informal way to say, "Goodbye," (like "Peace" or "Later") and does not have the same meaning as "get well."

I think "get well" implies that the person is not well now and needs to become well, whereas "be well" implies that the person is well and can continue being well.

I personally would most likely say, "(I) Hope you feel better" or simply "Feel better." This is entirely a personal choice.

Air conditioners sell well in the summer months.

Air conditioners are sold well in the summer months.

Those two sentences have slightly different meanings. "air conditioners sell (adverb)" describes if they are selling. "air conditioners are sold (adverb)" describes the skills of the person selling them, or the method they are using to do the selling.

With that said, "Air conditioners are sold well in the summer months" does not work. You could say "Air conditioners are sold well by the sales team", although it sounds a little constructed. You would probably want to use a present voice construction. "Our sales team does a great job of selling air conditioners during the summer."