The merchandise of many roman merchants did not sell because

The Romans traded around the world with their merchandise such as pottery, arts, foods, and book.  But trading all throughout the world was also the reason why the merchandise of many roman merchants did not sell due to the competition from each other and no proper trading technique made.

the answer is: must pay taxes on profits

since it is considered as a part of your income, profits from investments is eligible for tax payment.

the tax from your investment could be influenced by how long you decided to sell it. united states promotes keeping your investment for long term period. so, the tax rate for investments that you sell in less than one year is typically higher compared to investments that you sell in more than one year period.