$2 x 125 does equal $250.
Let's take that $250 - Ebay Fees - Paypal Fees - Postage - Material Costs.
I'd estimate that you'll clear $100. If that takes you 8 hours, then that is $12.50 per hour.
$12.50 per hour x 40 hours per week = $500 per week
$500 x 50 weeks a year = $25,000 per year
$25,000 - 30% self-employment tax = $17,500 per year
If you can do 125 in less than 8 hours, which you should, or you can scale the business, then it might get interesting. Hire some part-time helpers and grow the business. I personally don't see how anyone can have tens of thousand of decal designs without breaking all kinds of trademark and copyright laws.