HelenAnn - I started out getting most of the shuttles with the machines. I didn't originally play on getting a display case. But I did want some "extras" and needles. I spend too much time on eBay- at one point probably 2-3 hours a day (wasn't getting any sewing done!) and found a couple of bulk lots with filled tubes and long bobbins in clips.
The display case does not hold all that much of any one item, so I would figure that retail establishments kept a larger supply on hand to restock the display, and believe some of what I got was from restocking supplies.
And as we all know what happens once the ball gets rolling - I started paying attention to the cases, and got one at a very good deal that was about 3/4 full of supplies. I figured that was enough, but then a good friend found one at a good price, also about 3/4" full at a antique shop, so she called an asked if I wanting it. And for the price, I did. That one is older, and I'll put it in the Shed. The newer one I'll keep fully stocked and in the house.
If you can afford to make larger purchases - it's way more cost effective than paying $10 a shuttle to get them one at a time. However then you also have to want a wide assortment, very few people do. Often it would not be cost effective to buy in bulk to sell, as most of what you'll get will be the odder ones, and very few people are looking for them.