Regarding PayPal I'll totally agree. However i didn't think that Ukash is reversible.
I believe it is kind of like a first-to-spend situation. But after that, if the person you bought the code from stole it, the merchant may have trouble getting the funds that you redeemed with them.
If that happens, the may take action against you (e.g., cancel an order, recall a shipment, cancel subscription) or if they don't have an easy method of recourse, pursue you through legal channels.
Since UKash isn't supposed to be used person-to-person, using it in that manner you cannot claim innocence.
Ukash is not liable for the validity of any voucher codes originating from any person who is not a Ukash contracted business partner or for any losses or damages resulting from the supply of voucher codes from any such persons (whether such voucher code is supplied direct to you, acquired you in the manner described in section 2.2, or is the original source of voucher codes subsequently derived from that voucher code).