When they are issued, they can generally only be used by the members who receive them because the codes are linked to the email address to which they were sent.
This means that if you try to use someone else's coupon, it will in all likelihood be rejected at the checkout. If you try to use a voucher the was issued to someone else and it does fail at the checkout, you will still be obliged to pay for the item that you committed to buy (at the full price).
There are sites that offer various discounts, but these are not vouchers issued by either eBay or PayPal.