Who are the earliest fencers you can sell to? As early as the start of the game.

There are no fences to sell to until you've gotten deep into the Thieves' Guild questline.

Tonilia is the first and only until you get deep into TG side quests.

It might also be Macchius, at the Honningbrew Meadery, depending on if you start the main TG quest.

Get the perk in Speech or do the special TG questlines. You do get Mallus at Honningbrew and Gulum-Ei on the main TG quests, and after doing enough special missions in each hold you'll do a delivery of moon sugar from Tonila to Ri'saad making Khajiit caravaneers fences. I think Enthir at the college fences regardless of the TG, but it might be you get him when you and Karliah go to him in the main TG quest.