Plugins are one of the four parts of the WordPress system. Most WordPress self-hosted websites and blogs will have at least a few. There are free plugins and premium (you pay for them) plugins available from many sources. You really should consider the source.
Can You Resell Premium Plugins?
While it is legitimate in most cases to resell premium plugins, you are best served to purchase them directly from the author or original source, or from a well-known authorized reseller like Theme Forest or Gethub, and others. Purchasing this way most often comes with extras and/or premium service. Buying from an unauthorized reseller seldom include those things and may include things you don't expect, and don't want. See below...
And while their are many legitimate sources of free plugins, unless you are sure of the source, you are better to get free plugins from the WordPress repository (repo). You can search it directly from the "Add new" page in the Plugins section of your WordPress website's Dashboard.
Plugins obtained from unreliable sources often have back doors, trojans, malicious code, and other creepy crawlers that you probably don't want on your website. Some are just a nuisance, but some can cause serious harm to your website and your reputation.
Common Sense Plugin Sources
Common sense is a powerful weapon here, but good sense isn't always that common, is it? Choose your plugin sources wisely and you won't likely have too much problem.