First thing's first - it would be pretty difficult for your to actually take the payment first and then use it to buy the product from the supplier.
The process with eBay is that Paypal actually places the payments on hold for a minimum of 24 hours and wait for any possible dispute or the like. If there are no issues, it goes through.
This is actually one way to ensure that customers are also able to receive their products and establish a good sales front for your and your business.
There are several reasons you might not have immediate access to funds. Some of the most common reasons are:
You have a limited selling history.
You have a Below Standard rating in your Seller Dashboard.
If you are selling on eBay USA or Canada, your buyer's payment will first show as pending in your PayPal account if you have:
Limited selling history, meaning you have:
90 days or less from your first sale, or
25 or fewer transaction, or
US$250 or less total sales since your first sale
Below Standard Rating in your Seller Dashboard.
Unusual account activity such as sudden incidence of Buyer Protection case.
I suggest you try browsing through our eBay guide to learn further on policies - Link hidden: Login to view
Hope this helps.