So I got bored and decided to try my hand at finding out what the issue was. The 219x235 is the size of the base game storage tank file that the black market's buy/sell tank inherits but his graphic size is different (440x216). I went into the entity.lua file to see if I could reconfigure it and I got it to work by adding two lines under the part of the code that locates the PNG with tank in it:
tank_sell.pictures.picture.sheets[1].filename = "__MegaBlackMarket__/graphics/trading-tank-sell.png"
tank_sell.pictures.picture.sheets[1].width= 220
tank_sell.pictures.picture.sheets[1].height= 216
tank_buy.pictures.picture.sheets[1].filename = "__MegaBlackMarket__/graphics/trading-tank-buy.png"
tank_buy.pictures.picture.sheets[1].width= 220
tank_buy.pictures.picture.sheets[1].height= 216
Width is 220 instead of 440 because the tank has 2 configurations. Adding those two lines makes it work (But the graphic is a little jank if you zoom into it).