発売 and 販売: the start of sales and the selling activity
Compare 発売予定 and 発売日 with the activity expressed by 販売する. A planned release next month does not establish present availability.
A planned start of sales
発売, read はつばい, refers to a release or start of sales. 新しい辞書は来月発売される予定だ。 says that the new dictionary is scheduled to go on sale next month.
来月 and 予定だ mark a future plan. The planned start does not report a completed release or establish that the dictionary can be bought now.
The activity of selling goods
販売, read はんばい, means selling. 店で手作りの菓子を販売する。 says that handmade sweets are sold at the shop.
This sentence identifies the goods and selling activity. It supplies different information from 発売日, which identifies the start-of-sales date.
An action on the release date
限定切手の発売日に郵便局へ並んだ。 says that someone lined up at the post office on the release date of the limited stamp. Lining up is reported in the past.
A product can have both a release and subsequent selling activity. Read the time focus without treating 発売 and 販売 as mutually exclusive events.
Does 来月発売される予定だ。 say that sales have already started?
Show the answer and why
No. It describes a planned start next month, without establishing present sales.