Question:
If I’m having a yard sale and selling many products, do I have to give maaser (tithe) from the income I make from the sales?

Answer:
You only need to separate maaser from sold items if you made a profit. This means that if the items originally cost you a certain amount and you sold them for more than that amount, you should take maaser from the excess. However, in most yard sales, you typically receive only a fraction of the original price you paid. Therefore, it’s considered as simply recouping some of the initial money you spent. Since maaser was already taken from the original income used to purchase those items, the money from the yard sale is generally exempt from maaser.

For instance, if you bought a blender for $100 and later sell it at your yard sale for $20, you don’t need to give maaser because you’re just recovering part of your original expense. However, if you purchased a collectible for $50 and sell it for $80, you made a profit of $30, and you would give maaser on that profit, which is $3.

Sources:

Igros Moshe Y.D. 2:114; Igros Kodesh 18 p. 384, printed in Shulchan Menachem 5:88; Pesakim Uteshuvos 249:25 footnote 239