I too like to have a range of clothes, I think it does help with the selling rate. I've been playing since the game come out.
When, I first started playing I tried to build up lots of quick stock items and then more longer term ones, it does get easier to maintain a good stock level, once into middle levels. I think we do lose money when we change an order as we have to cancel and spend money on a new order. Some players block the door or cash register, however it can result in a quick sale sometimes and all stock may go. I've found it helps to visit nbrs after unblocking the door, so it may help prevent the quick sale. Getting your 20 gifts a day also helps with stock levels.