If someone buys your Fixed Price listing, you will get an email from eBay.
If someone bids on your Auction listing, you won't, but when the auction ends, you will get an email from eBay.
When the buyer pays, you will get an email from Paypal.
ONLY look at your Paypal account for proof of payment.
Your customer's payment will be Held until your customer receives the item.
If you mean, how do I know if anybody will ever want to buy my product, no one really knows that until it is listed and someone buys.
Although you can research what has sold in the past and how much it sold for.
You have up to 200 Free Listings, so making your product available will not cost you anything but your time.
There are fees when you sell.
