If you want readers to buy your series, you have to attract them to your book page in the first place before they can decide whether or not to press that 'Buy' button.
Your cover is vitally important because it is the only part of your book that will be seen by potential readers on other parts of Amazon's site...in the Also Bought lists of other books and in Listmanias. So your cover is out there attracting the attention of readers as they browse other areas of Amazon.
Your cover should make it clear what the genre of your story/series is. Most readers know what genre they like to read in so make it easy for them. Your cover should say, 'Yes, this is the genre you are looking for.' Don't be ambiguous.
Take a look at these covers. Is it obvious what genre they belong in?
Take a look at Sara Fawkes' 'Anything He Wants'...does the cover leave you in any doubt that this is a story about steamy sex? The title itself hints at the domination theme. A sci fi reader seeing this cover on Amazon probably wouldn't be tempted but a reader looking for a sexy story with a dominant male would see this and instantly recognoze it as what they are looking for.
Parnell Hall's 'Detective' lets us know it is a mystery by the title (what do detectives do if not solve mysteries?) and the subtitle 'A Stanley Hastings Mystery. The picture, along with the title and subtitle, leaves us in no doubt that if we are looking for a mystery novel, this is a good candidate.
My own 'Ghost Dance' is a Western, and the horse and rider place it firmly into that genre.
H.P. Mallory's 'Great Hexpectations' cover lets us know that the story inside is a paranormal tale told with humor. The titles play on words and the whimsical illustration leave us in no doubt about the humor part. The cover also clearly places the book in a series and lets readers know it is number three in that series.
'Haunted' by Willow Cross leaves readers in no doubt that it contains horror stories. An atmpospheric, dark picture of desolate trees with a skull and creepy figures in the foreground as well as the words 'haunted' and 'ghost' leave the reader in no doubt what is contained within.
When you decide on covers for your series, think about the visual clues you are giving the reader about what lies within the covers of your book. Both the images you use and the words on the cover combine to create an idea in the readers mind. Make sure that idea is the correct one. I've seen some covers on Amazon that have nothing to do with the stories behind those covers. The authors (or their cover designers) did ot use visual shorthand correctly.
Now let's look at branding. Just as the cover of 'Great Hexpectations' above clearly shows that the book belongs in a series, you will probably want to link your covers in some way to show the same thing.
There is a basic human drive to collect things. That is why stories in series sell better than standalones. Use that basic human drive to your advantage...clearly show readers that each of your stories belongs to a greater whole. You might use a specific color scheme on all the covers, or a logo for the series, or a colored bar down the edge of the cover with the series name in a specific font.
What you are aiming for is uniformity. Even though each episode must look different and have a picture that is unique, they should all have some uniform element that links them. If a reader sees a row of books on an Also Bought list, how much more appealing are those books if it is clear they are linked together in some way?
Think about your genre. Think about your brand.