Why do economists regard imperfect competition as undesirable?

Imperfect competition is not necessarily viewed as undesirable, but it is viewed as different than perfect competition. Perfect competition is defined by a very strict set of rules that include having no barriers to entry and no single business having the ability to influence price. 

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.

d definitly is not