Say what you actually know
If you have a date from your supplier, share it as an estimate. If you do not, say you are not sure rather than guessing.
Offer a notify option
Point them to a back-in-stock notification or add them to a list. It saves them from writing again and tells you what to reorder.
Suggest a close alternative
If a similar item is in stock, link it and say how it differs. Keep it to one or two options.
Do not promise a date
A missed date costs more goodwill than an honest maybe. Use words like expected and not guaranteed.
Use requests to plan stock
Count how many people ask for each item. Frequent requests are free demand data for your next order.
Close the loop when it returns
When the item is back, tell the people who asked. A short message often turns into a sale.