The sell through rate is the percentage of listings that have sold. eBay store listings will generally have a much lower sell through  rate than regular listings so it is probably best to do two separate sell though rate calculations with one for eBay stores and one for regular listings. The formula looks like this:

Listing sold / Number of listing = Sell through rate

Example:

100 Listings, 40 listings Sold

40 / 100 = 0.4 or a 40% sell through rate

If you don't run dutch auctions or multiple quantity fixed priced listings this is the same as your sell through rate and there is no need for you to track it. Just like the sell through rate you should exclude eBay store listings for this number or run two separate calculations.

The formula is similar to the sell through rate and is:

Number of items sold / Number of listing = Number of items sold per listing

Example:

100 Listing, 55 total items sold

55 / 100 = .55 items sold for every listing.

You could also view this as a percentage of 55% to make it identical to the sell through rate.

You would want to track because a item with a sub par sell through rate of say 25% would still be doing well if this number is high. For example .45 items sold per listings is roughly equivalent of a 45% sell through rate.

Gross margin is simply how much profit you make. Historically gross margin uses just the cost of your item and not other things like eBay and PayPal fees however if you want figure out exactly how much you are making on a item after eBay fees and PayPal fees you can add them to your cost or you can do two separate calculations to figure your margin before and after fees.
Gross margin

Gross margin as a percentage:

(Sale price - Cost) / Sale price = Gross margin

Example:

$9.95 sale price, $5.50 Cost

($9.95 - $5.5) / $9.95 = 44.72% Gross margin

Gross Margin in actual dollars:

Sales Price - Cost = Gross margin

Example:

$9.95 - $5.50 = $4.45 gross margin

Gross margin can also be used to determine how much to charge for a item. If you have a item you paid $5.00 for and want a gross margin of 60% you would use the formula below.

Cost / (1 - Gross Margin Percentage)  = Selling Price

Filling in the numbers you get

$5.00 / (1 - .6) =  $12.50. So in this example you would need to sell the item for $12.50 in order to have a gross margin of 60%.

This is a percentage of how much of your sales you are paying to eBay.

eBay Fees / Sales = Percentage of sales you are paying to eBay.

Example:

$800 in sales, $64 in eBay fees

$64 / $800 = 8%

Unlike the other numbers you should include your store listings in this calculation. This number provides a great overall snapshot of your business and tells you just how much it is costing you to generate every sale on eBay. As with the other formulas you can choose to include PayPal fees with the eBay fees, it really doesn't matter just be consistent. If you include PayPal fees in one formula and don't use them in another or use them one month but not the next you will the numbers will not be comparable.

Fees / Number of sales = Cost per conversion

Example:

100 Listing, 55 sales, $64 in fees

64 / 55 = $1.16 cost per conversion.