Profit is measured on revenue, not revenue minus miscellaneous costs.

A
B
C
1
Sell-Based Percentages
2
eBay
10.00%
B2: Input
3
VAT
0.00%
B3: Input
4
PayPal
2.90%
B4: Input
5
Margin
15.00%
B5: Input
6
Fixed Costs
7
Post & Packing
$0.87
B7: Input
8
PayPal
$0.20
B8: Input
9
Cost
10
Direct Cost
$1.72
B10: Input
11
12
Sell Price
$3.87
B12: =(B10 + SUM(B7:B8)) / (1 - SUM(B2:B5))
13
14
Check:
15
eBay
$0.39
B15: =B2 * B12
16
VAT
$0.00
B16: =B3 * B12
17
PayPal (variable)
$0.11
B17: =B4 * B12
18
Post & Packing
$0.87
B18: =B7
19
PayPal (fixed)
$0.20
B19: =B8
20
Your Net
$2.30
B20: =B12 - SUM(B15:B19)
21
Profit
$0.58
B21: =B20 - B10
22
Margin
15.00%
B22: =B21 / B12