Once you had won the item and had paid for it the item subsequently became your property, so the seller had no right to relist it for sale again just because he didn't get as much as he wanted for it. It's the seller's responsibility to start the bidding off at a price that would ensure that he got the minimum amount that he wanted for it in the event that he only got one bid.
There are a few things that you can do to deter the seller from doing this again. First of all, make a note of the item listing number and the seller's User ID, then contact eBay Customer Services and explain to them that the seller cancelled the sale and refunded your payment without your consent and then relisted your item a second time. Point the eBay representative to the message that the seller sent to you where he admitted that he didn't get as much for them as he wanted and so he has relisted them again. If you do this then eBay may well decide to monitor the seller's account in case this is not just a one-off occurrence but instead turns out to be a regular habit of the seller's if he doesn't get as much as he wants.
With regards to your question "Is there a method to warn others of this kind of misuse?" the only option to do this is to leave the buyer some negative feedback in relation to the transaction. Something along the lines of "Won auction and paid straightaway but seller refunded money and relisted item" would suffice, as there is nothing in that comment which would breach eBay's rules as to what you can and cannot say in feedback, and therefore the seller would not be able to get the comment removed, especially if you'd already reported him to eBay for reneging on the deal and relisting the item for sale without your consent. If you did leave some feedback for the seller similar to the kind of comment that I suggested then it may well serve as a warning to other potential buyers and put them off of purchasing anything from this seller.
