Hi, unless you define what you want to do there's no great answer.
Pages is inexpensive and you probably already have it. If you just want big letters you could even use TextEdit which you will have for sure.
Assuming you want a for sale sign that looks like the ol' red type on black box with a white area for writing-- well you could accomplish that with loads of apps such as Pages, Numbers, Word, Excel, Illustrator, InDesign, XPress... (the list goes from really cheap to really expensive). You could even do it in a database to automatically change the details (Bento, FileMaker).
Then there's PhotoShop which could be used.
As a guy coming from a print background I'd probable use Illustrator, InDesign, or XPress. That being said you can use just about any app that prints to make a simple for sale sign.
If you define what you want I can probably get specific.
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