Do I give Maaser if I sell something I bought with money I already gave Maaser on?

You need to give Maaser only on the extra profit from selling the product. For example, if you bought it for $10 and then sold it for $11, giving you a $1 profit, then your Maaser would be 10 cents.

 

Sources:

.שו”ע יו”ד סי’ רמ”ט ס”א