Don’t open that controller yet!
I worked for eBay once upon a time, and I’ve gone through this process before for myself.
Double-check that the eBay listing does not specifically mention 1) that the R button is mushy, or 2) that the controller is “sold as-is” or that the purchase is “non-exchangable.” Assuming these criteria are not met, you have a legitimate complaint and are elligible for a replacement or refund. If the seller did cover his ass as described above, he was probably looking to rip somebody off and unfortunately you had to be that chump. Chances are, though, that he didn’t mention any of those things.
Don’t leave the seller negative feedback (yet). Remember he’ll probably leave you a negative feedback too. You have a full 90 days to do this from the date of purchase so there’s no rush. Wait until the situation described below is completed before bothering with feedback.
Open an “Item not received or significantly not as described” dispute. (If it asks you to specify, you are the latter category, of course.) Specify in your description of the situation thus far that you have not yet attempted to contact the seller, that it feels like the spring in the R shoulder button of the controller is broken, and that you’d like a replacement if possible, or a refund if not.
Mention that you’d like to avoid leaving negative feedback if at all possible. If it’s a big eBay business, it wouldn’t hurt to drop in some phrases like “Don’t worry, I understand that mistakes happen.” The seller will probably bend over backwards for you.
Once the dispute has been created, you’ll have to wait for the seller to respond. Check it every couple of days (through the “Dispute Console” in your “My eBay” page) to see if he’s answered yet.
Just respond honestly and provide any additional information as needed. I’m afraid the process can take a while, but remember that it IS working for you.
Be polite and professional. Trust me on this! The more you sound like an asshole, the less eBay wants to help you. The more you sound like a perfectly nice guy, the more eBay wants to help you. Even if the other guy is clearly a jackass, just don’t go there. It is to your benefit and he’s only screwing himself over.
If you’re following all my above advice, 99% of the time the seller will painlessly give you exactly what you want. He might ask you to send back the old controller: do so under the condition that he pays you back for the cost of shipping. Don’t close the dispute before everything has been taken care of: make sure the replacement controller works properly, make sure he paid you back for return shipping, whatever. Once everything has been settled, thank him and close the dispute as resolved.
If the seller absolutely refuses to accomodate your requests after a couple of reasonable attempts by you on-record in this dispute, escalate it to an “Item significantly not as described” claim. eBay will bring the hammer down on his ass and you’ll have your refund processed.
Worst case scenario, he doesn’t respond to the dispute at all. Eventually, the dispute will be automatically escalated to a claim and ass-hammering proceeds as described above.
(You can leave whatever feedback you deem appropriate now. I guarantee the seller will mirror whatever you gave him.)
It might take a couple of weeks but everything will work out in your favour.
If the seller told you to just keep the broken controller and you still care about fixing it, by all means follow everyone else’s advice.
Hope this helps! 
