This is a very common mistake, as known as a misconception of switching lights on and off, by making that detects if it's true, on the light. What are you currently is doing is passing two statements, when the Sell is disabled, it goes to the if statement of detecting if the Sell is disabled, and then switch it true. If your default setting had Selling Disabled true, then it pass through the if that sets the Disabled false and passed through another statement and set it true.

Anyways, you can use if-else with this.

script.Parent.MouseButton1Click:Connect(function()
    if game.Workspace.SellingPart.Sell.Disabled == true then
        game.Workspace.SellingPart.Sell.Disabled = false
        game.Workspace.SellingPart.Sound.Disabled = false
    else
        game.Workspace.SellingPart.Sell.Disabled = true
        game.Workspace.SellingPart.Sound.Disabled = true
    end
end)

If this doesn't work, make it as a LocalScript and insert this instead:

wait()

script.Parent.MouseButton1Click:Connect(function()
    if game.Workspace.SellingPart.Sell.Disabled == true then
        game.Workspace.SellingPart.Sell.Disabled = false
        game.Workspace.SellingPart.Sound.Disabled = false
    else
        game.Workspace.SellingPart.Sell.Disabled = true
        game.Workspace.SellingPart.Sound.Disabled = true
    end
end)

NOTE:

DONT PUT THE SCRIPT/ LOCALSCRIPT IN WORKSPACE OR SERVERSCRIPTSERVICE, PUT IT INSIDE THE TEXTBUTTON.