And, technically, you are breaking Ebays TOS being a minor and selling on Ebay. I would imagine that means you also fibbed on your PayPal account.
This online store you want to buy equipment from would be extending you credit. That means, you need to be 18 to even apply. At 17, you cannot enter into a binding contract. Even IF you lied on the application ( as is it seems you did to Ebay and PayPal ) about your age, you would still be declined to make payments on this equipment you want.
If you use your debit card, you need to have $500 to pay for the entire purchase. A debit card is NOT a credit card. Even if it has a MC or Visa logo on it. If you were to enter your debit card info, they would charge your account $500. That means, it would come out of your bank account when you make the purchase.
I find it funny you say how you don't want a credit card, but yet, you are ok buying this equipment on credit. LOL