Looks like your savedata got corrupted.
You can try to edit MasterMerchant.lua in the addon directory and modify InitPurchaseHistory (line 2749) to look like this:
Lua Code:
local item = dataList['sales'][i]
if(item.guild) then -- add this
self.guildPurchases[item.guild] = self.guildPurchases[item.guild] or MMGuild:new(item.guild)
local guild = self.guildPurchases[item.guild]
guild:addSaleByDate(item.buyer, item['timestamp'], item.price, item.quant, item.wasKiosk, false)
end -- and this
or if it doesn't help delete all master merchant related save data (SavedVariables/MasterMerchant.lua and all MMxxData.lua).
Either way you will loose parts or all of your trade history and it will have to regenerate it.