I believe the word you are looking for is profiteer which meriam-webster online defines as

profiteer

noun prof·i·teer ˌprä-fə-ˈtir

: one who makes what is considered an unreasonable profit especially on the sale of essential goods during times of emergency

I PROPOSE THE INVENTION OF A NEW TERM FOR THIS DEFINITION: n. SHKRELIST (REFERENCE: MARTIN SHKRELI, CEO OF TURIN PHARMACEUTICALS) V. TO SHKREL

"Profiteer" is perfect to answer to the question in the title.

In the context of the example, i.e. "coffin seller", the words bloodsuckers (or leeches) and scavenger (or vulture) may fit, if you want to be more literary.

I believe in this instance the word you need is Vulture. Vultures are found in nature feeding upon the dead and dying and thus gaining life at the expense of others. If you wish to convey someone who feeds upon the living at their expense a closer match would be Vampire. Both of these words have been used to refer to people who profit from the suffering of others.