Move in y-axis until user stops holding down button?

Replace your Original Stage trigger action code with this; Comments is appended to the code;

/* Your Original Code
sym.getComposition().getStage().getSymbol('lab_items').$('btn').bind('mousedown touchstart',function(e){
sym.getComposition().getStage().getSymbol('lab_items')
.getSymbol('numbers')
.$('labMarks')
.animate({'top':'-=50px' }, 300, 'linear');
});
End Your Original Code*/

///////// New Code   //////////

// Use a function to hold the continuous scroll animate method.

// The trick here is with the last parameter scrollCont.

// Which essentially calls the function (itself) over and over,

// looping, until mouseup is triggered later

function scrollCont(){
    sym.getComposition().getStage().getSymbol('lab_items')
.getSymbol('numbers')
.$('labMarks')
.animate({'top':'-=50px' }, 300, 'linear', scrollCont);
}

// Function to stop() the continuous scrolling when

// mouseup is trigger, later

function stopScroll(){
    sym.getComposition().getStage().getSymbol('lab_items')
.getSymbol('numbers')
.$('labMarks').stop();
}

// Your mousedown touchstart still applies; the addition event

// chaining the mouseup touchend; which

// when triggered will fire the stopScroll() function.

sym.getComposition().getStage().getSymbol('lab_items').$('btn').bind('mousedown touchstart',function(e){
    scrollCont();
}).bind('mouseup touchend', function(e) {
    stopScroll();
});

////// End New Code ///////

hth

Darrell