This seems to be pretty common in HK. I have never attended a violin class, but did attend some other classes (like sewing classes), and usually the instructor would have something to sell you, or offer to buy stuff for you. Usually they would tell you the price, and will not give you receipt for that. Sometime it is convenient, cause it saves you time, and you don't have to find where to buy the stuff that you need.... I guess the teacher also makes a profit out of the "favor", or get some sort of reward from the place she gets the merchandise.
I think it is acceptable for cheap stuffs... but for a 4K to 5K violin, I would really do some research first, specially since there are many big musical instrument shops located in convenient malls, were you can go an buy the violin by yourself. You can also ask you teacher about the model, and if it would come with any guarantee (i guess if it comes with a guarantee, then it will come with a receipt). Then check for the price and see if it really worth for you buying it from your teacher.