If Madara took Obito's left eye, how come he has both Sharingans? (Chapter 683, page 15) Did Obito replace the one he lost with Kakashi's Sharingan, like a trade?
Because Madara gave him the Sharingan he had taken from Kakashi back when he took the Rinnegan Obito had in his left eye socket.
In chapter 674, page 14, Madara takes Kakashi's Sharingan eye (Obito's left eye):
In chapter 675, page 10, Madara switches his left eye - currently holding Obito's left eye - for the Rinnegan Obito has in his left eye socket:
In that same chapter, he lets Black Zetsu take control of Obito's left side. In chapter 676, page 2, we see that Obito now has both of his Sharingan, though he is not in control of his body:
In chapter 679, page 6, Black Zetsu leaves Obito's body to join Kaguya:
Leaving him finally in control of both of his Sharingan when Naruto revives him and he regains consciousness in chapter 683, page 6:
Obito had his Right eye Kamui Sharingan and Madaras Rinnegan in his left socket. Madara snatched Kakashis Kamui Sharingan and used it to enter Kamui Dimension and retrieve his Rinnegan. He was successful and Black Zetsu possessed Obitos right body half, which was eyeless until Madara popped Kakashis Sharingan in. Obito got both his eyes back in that moment, but B Zetsu was using the Obitos right half and used Kakashis eye to get himself and Madara out of the dimension.. It was mentioned, but it was easyly possible to miss.. I thought for quite a while that Obito didn't have both of his eyes until it was cleared up to me, that the Obitos right eye was never touched by madara.. thus giving him both eyes. Why did Madara pop Kakashis Kamui eye back? I don't know... Lets just say for plot, because he could have just crushed it. I'd assume he didn't consider Obito a threat anymore and wanted the MS Sharingan as backup for whatever reason.. It really caused a stir here when those chapters came around
The "unmentioned" situation you're looking for spans from chapter 674 to 676.
This is the answer given by that person, you can refer to his post here. http://forums.mangafox.me/threads/511150-How-did-Obito-get-his-left-eye-back
He gave it black zetsu to use madara and himself to get out of the dimension madara basically madara put his rennigan on while he was in the other dimension and only way to get out is with obitos sharingan capish
Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.