Why does one product cost differently from different stores online?

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Amazon quotes me one price for a deep fryer, 3 piece, stainless steel stove top with glass lid and wire basket and the same exact product sells for about 10 dollars more at Walmart and eBay has yet another price for the same item. So, I am not only confused by the price differences, I am confused because Amazon lists a product twice that appears to be exactly the same one for 2 different prices. If this is because one of these items only looks the same but has some differences in size or material. I can't be sure. The details on all of these vary. In one case, the product gives me the model number, in another display it tells me how many quarts it can hold and in all cases there are extra charges like, 1 day delivery or 2 day delivery or this that and the other. I get so confused and to add insult to injury, I have to divide up the amount to pay for it by first paying with a gift card, that is 25 dollars to buy the product but the extra tax adds to the price of the item. I have then to pay the difference by using another credit or debit card and add them to my list of payment methods because they are not mine so I just told my wife, "PLEASE WAIT UNTIL I HAVE ENOUGH IN THE BANK TO PAY FOR IT MYSELF." It is much simpler for me to go to Walmart and purchase it in the store than go through hoops to pay for it online and sit and wait for it to arrive at the doorstep. Don't you think online can sometimes be a PAIN? To get to the root of this matter I realized after seeing her Kindle. I bought her Kindle from Amazon. I have the account in my name. Amazon posts the product on her Kindle because I was looking at it on my PC. Because she has the Amazon app, she can see my wish list and worse, I can't keep it a secret because the ads that they put on her Kindle reflect MY VIEWING HISTORY. I wanted for it to be a surprise but now she thinks she doesn't even want it and is looking for another brand. She thinks just because the price dropped by about 5 dollars and says BUY THIS WITHIN SO MANY HOURS before the price goes back up, is like putting a carrot in front of the proverbial race horse. I came across this very significant post about why a particular online seller, has different price quotes for the exact same product at their site, sometimes, not always: