To be honest I would advise you to go to a professional to get the housing replace. I repair iPhones then sell them and specifically the iPhone 3GS right now, so I now what I'm talking about.
However it is very possible to fix it yourself but you would have to follow YouTube videos exactly and they have a 10-20% vagueness to them.
Also be careful if you want to buy replacement housings from eBay and online just about all of them are counterfeits from China (I'm not certain on that but I'm pretty sure on this. I've bought so many and every single one has been a copy) some are good quality and some are bad quality. Also it is illegal to knowingly purchase counterfeit items.
The best place to buy replacement housing is WHAT ME MYSELF AM TRYING TO FIND OUT. I HAVE SEARCHED EVERYWHERE AND I HAVE NOT FOUND ONE PERSON SELLING AN ORIGINAL APPLE IPHONE REPLACEMENT HOUSING. But I will say try an Apple Certified Technician like TechRestore or iResQ (just Google them) they can do the repair but Im almost certain that they do not have ORIGINAL PARTS.
If you said that you only pushed in the lock button then you would not need to replace the mute/power button. All you would have to do is realign it. There are few small screws used to install this component and if you don't have the right TOOLS and don't know what to do you will probably mess up. Plus you have to open the phone up. I nearly died on my first attempt at fixing a 3G.
Seriously though it is very doable for your first attempt just try to follow the YouTube videos as best as possible (remember 10-20% vagueness). And your tools should almost certainly be magnetized its critical.
Concerning the eBay cases I would sell 1 at a time but let buyers know that there is more than 10 available that way people can just continue to buy them until all 50 is gone. Pictures basically sell your product on eBay make sure you have clear pictures and you may just sell all of them.
OH AND BY THE WAY IF ANYONE KNOWS WHERE I CAN GET ORIGINAL IPHONE
3GS BACK COVERS (NOT REPLACEMENTS BUT ORIGINALS) PLEASE SKYPE ME. I HAVE SEARCHED ALL OVER THE INTERNET.