Put the table and the art board on your property, click and choose put in inventory. go to where you want to sell your stuff. open inventory and choose, place in world. once placed click on it and choose stock from inventory, like the fridge another pop up screen appears. drag what you want to sell to the pop up. then click table and select stock, tend table, start street sale and mark up prices in that order.
be careful though because sometimes another sim will come along and take over your table or art stand. but you should still get the money. and remember to put your stuff back in your inventory before the flea market finishes.