[Nexus 5 Q&A, Help & Troubleshooting] Phylum:
This post is for two audiences:

For those who have replaced their Nexus 5 battery: Where did you get it from and are you happy that its a legitimate replacement?

For those who buy Nexus 5 accessories, can you recommend reputable vendors that either do or might sell Nexus 5 batteries?

The only vendor that keeps popping up in the forms is ETradeSupply (https://www.etradesupply.com/lg-nexus-5-battery.html). They're currently selling Nexus 5 batteries (BL-T9) for $13.97 (down from $15.99), but the $20 shipping, since its coming from Hong Kong - which is also somewhat of an annoyance because it could be a while before I get it - it made me look elsewhere.

eBay

I did a quick search on eBay for new items located in the US Only and scoured the results. The list below only includes sellers with

high positive feedback percentages,

high feedback scores,

had photos like the real-deal

Yes, I know they're just photos and can easily be altered or copied from elsewhere etc.. But there are several who have blurred out the LG Logo and name, or are selling batteries that look like the LG one but don't have the logo (and don't appear to be altered - here's an example). There must be a reason for that, so I personally wouldn't consider them.

So here are some hopefully reputable eBay sellers that I'm eye-balling at the moment:

eforcity - 99.4% positive feedback with a feedback score of over 4 million: http://www.ebay.com/itm/Genuine-OEM-Google-Nexus-5-LG-D820-D821-2300mAh-Battery-BL-T9-3-8V-8-74Wh-New-/391054249386? & http://www.ebay.com/itm/Genuine-OEM-Google-Nexus-5-LG-D820-D821-2300mAh-Battery-BL-T9-3-8V-8-74Wh-New-/391054249386?pt=LH_DefaultDomain_0&hash=item5b0ca64daa

silicon_electronic - 99.4% positive feedback with a feedback score of 218K: http://www.ebay.com/itm/New-2300mAh-Replacement-Battery-for-Google-Nexus-5-LG-D820-D821-BL-T9-BLT9-/231306676152?pt=LH_DefaultDomain_0&hash=item35daf3c7b8

gadgetfix - 98.8 positive feedback with a feedback score of just over 250k: http://www.ebay.com/itm/USA-LG-Google-D820-D821-Nexus-5-Original-OEM-Battery-BL-T9-2300mAh-EAC62078701-/371267382445?pt=LH_DefaultDomain_0&hash=item567142acad

techflavour - 99.3% positive feedback with a feedback score of 191K: http://www.ebay.com/itm/Genuine-Original-LG-Google-Nexus-5-D821-BL-T9-Replacement-Stand-Battery-2300mAh-/331478953125?pt=LH_DefaultDomain_3&hash=item4d2daf6ca5

wholesalegadgetfix - 98.7% positive feedback with a feedback score of 148K: http://www.ebay.com/itm/USA-LG-Google-D820-D821-Nexus-5-Internal-OEM-Battery-BL-T9-2300mAh-EAC62078701-/171697435211?pt=LH_DefaultDomain_0&hash=item27f9f6f24b

phone_cellar - 99.2% positive feedback with a feedback score of just under 55k: http://www.ebay.com/itm/New-OEM-LG-BL-T9-BLT9-Internal-Battery-w-Flex-for-Google-Nexus-5-D820-D821-/201279704622?pt=LH_DefaultDomain_0&hash=item2edd348e2e

nabaisi_tech - 99% positive feedback with a feedback score of 24K: http://www.ebay.com/itm/New-Genuine-2300mAh-BL-T9-Battery-for-Google-Nexus-5-D820-EAC62078701-LLL-Tools-/131403306346?pt=LH_DefaultDomain_0&hash=item1e983f556a & http://www.ebay.com/itm/New-Genuine-OEM-Google-Nexus-5-LG-D820-D821-2300mAh-Battery-BL-T9-3-8V-8-74Wh-/131282556946?pt=LH_DefaultDomain_0&hash=item1e910cd812