You shouldn't post apple serial numbers publicly as scammers clone them, but using it I was able to determine that it is the MD101. Honestly, you can get a faulty laptop in anything. The other models that were available at the time you purchased were a few hundred dollars more AND no longer had an optical drive internally. Sure they had a retina display and a much faster processor, but Apple knew that keeping this around as a low-cost option was a sure fire profit maker. I'm sorry to hear about your trouble, but the information provided by the associate is correct. Other than the model identifier and info in the menu, there is nothing to indicate the length of time the model was sold. I do not believe there was anything malicious in offering this model to you.
Batteries do age and they do fail, especially in a laptop after many charges. There is no reason that this Mac would have sat in the store since 2012, this is not a reason for your battery troubles. Here are a few tips to determine what is going on with your battery life:
1. Let's make sure the battery is fine. Go to the Apple Menu > About This Mac > System Report, Then select power on the left column. You should see your cycle count and battery condition under Health Information. If the condition is normal, but your cycle count (number of times you charged it) is high then it could be the battery. Above that you will have your battery capacity and charge remaining. Charge it to 100% and go to this screen, unplug and see if it drops signifcantly on the charge remaining. If the two numbers are close, then there is nothing wrong with the battery.
2. At this point, I would look for a runaway or troublesome app using significant energy. An easy way to do this is to click the Battery in the Menu Bar and see if anything is listed as "Using Significant Energy". A more detailed way would be to use Activity Monitor. You can Command+Spacebar to search for it, or open it in Apps > Utilities. Once you've opened it, click the Energy Tab and sort by the Energy Impact column. Any offending apps should show here. You can uninstall and reinstall or contact the app manufacturer for assistance in correcting the issue.
3. If it's not the battery or an app, you can try resetting the NVRAM and/or SMC on the Mac. The steps are on support.apple.com. I would recommend reading the instructions and caveats for each. Some settings are cleared in doing so.
Hope this helps.
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