noun which may take the genitive case particle 'no'
on sale; available for purchase
Used for products, tickets, or items currently being sold or available to buy. Often seen in advertisements, store signs, and online listings.
このゲームは来週から発売中です。
This game will be on sale starting next week.
新商品がただいま発売中です。
The new product is now available for purchase.
販売中 also means 'on sale', but 発売中 emphasizes the initial release or availability of a new product, whereas 販売中 is more general for any ongoing sale.
Compound of 発売 (release, sale) and 中 (in the state of). The kanji 発 means 'departure' or 'emit', 売 means 'sell', and 中 indicates 'in progress' or 'currently'.