Amazon's fee for books is roughly $2 + 15% of sale price. The 15% fee is a little high, but there's no PayPal fee like with eBay payments, and it's worth it IMO. Amazon charges a bit less than the 15% for all items other than books. Fortunately, unlike eBay, they don't charge a listing price, so if your book doesn't sell -- as happens quite often, especially with textbooks that have suddenly become obsolete -- then you're not out any money. You can see a detailed explanation of Amazon's fees.
In order to be worthwile to bother buying and reselling something, you have to be able to profit from the affair. Since
Profit = Revenue - Cost
and we need Profit to be positive, we need Revenue to be greater than cost, or
Revenue > Cost
As stated earlier, Revenue is the amount you get for selling the item online minus the overhead that Amazon/eBay charges, minus whatever other overhead such as shipping you incur.
Let's look at the example of selling a textbook on Amazon. From the description of Amazon's fees the overhead for selling a book with Domestic Standard shipping is roughly $1 + $1.23 + 15% of sale price. In addition, although Amazon reimburses you $3.50 for standard shipping costs, I usually pay about $1.50 more, depending upon packaging materials and weight. In other words,
Revenue = 0.85 * sale price - $3.73
with the $3.73 being the constant overhead of $1.00 + $1.23 + $1.50. So, for Revenue to be positive, eighty-five percent of the item's sale price must be greater than $3.73. So if you already have a book (i.e. you don't have to buy it used), and you're wondering whether it's worthwile to sell it on Amazon, you must sell it for at least $4.39 or you're losing money.
Now, what if you're at a used book store and you're wondering whether you should buy a certain book. From before, we have
Profit = Revenue - Cost
with Revenue the same as before (from selling the book online, minus overhead) except this time we have a cost — the cost of buying the book. If the book is on sale at the bookstore for $2, then
Profit = Revenue - $2
Profit = 0.85 * (sale price) - $3.73 overhead - $2 cost
In order for Profit to be greater than zero, you must be sure you'll be able to sell it for at least $6.75 online. If you're not sure you'll be able to sell it, you could come up with a guesstimate of the probability that the book will sell, and take that into account for expected revenue.
If you've ever went into a bookstore looking for anything of value, and been astounded by all the books there that would sell for less than $7, now you know why. Book sellers with a brain don't bother with these small potatoes.
eBay is decent for most other stuff, like electronics. Just be careful if you're selling (or buying) anything pricey, as there are a fair amount of disputes and outright fraud that goes on.
Now, let's consider the slightly more complicated case of buying a pricey electronics item online for a nice discount with rebates, and reselling it on eBay for a profit. For this example, we'll use a deal posted to SlickDeals.net on June 17. The offer is for a Sandisk Sansa e260 4 GB mp3 player that's going for $120 after rebates and coupons. The original price was $200. After applying a $30 instant coupon code, a $35 manufacturer's rebate, and a $15 rebate, it costs only $120.
Well, not so fast. That's assuming we get all the rebates in. I trust Office Depot rebates, more or less. Let's be liberal and say we have a 90% chance of getting the $35 rebate back alive. I'm a little more skeptical of the $15 rebate from SanDisk, especially since there's only one UPC to mail in for two rebates, leaving us to photocopy and pray. Let's give SanDisk a 75% chance of giving our $15 rebate to us. We'll assume the coupon code has a 100% chance of working (i.e. we wouldn't go through with the offer if the coupon code didn't work at checkout). That leaves us with an expected price of:
Price = $200 - $30 coupon - 0.9 * $35 rebate - 0.75 * $15
Price = $127.25
Let's assume that you have excellent feedback, and that someone would be willing to pay slightly less than market value on eBay for a new SanDisk e260. Doing an eBay search for " Sandisk e260", it looks like people are willing to pay roughly $180, with shipping, for it on eBay assuming modest luck, decent feedback score, etc.
Here are the eBay Insertion and Final Value Fees, and PayPal fees. Someone's put together a handy Ebay/Paypal Fee Calculator that will do the work for you. Using the calculator above, a starting bid price of $1, and a final sales price of $165 + $15 shipping, it looks like eBay/Paypal skim a total of $11.38 off the whole shebang. In other words, our Revenue from eBay comes out to:
Revenue = $165 - $11.38
Revenue = $153.62
While the total price we paid for the item, with only $1 overhead, was $128.25. We can now calculate our expected profit,
Profit = Revenue - Costs
Profit = $153.62 - $128.25
Profit = $25.37