I think that using a disposable mascara wand will be ok because you can get the general idea of the formula of the mascara but it is not ideal. This is because the shape and texture of the wand has so much effect on how your lashes turn out. Different wand shapes are better than others and the disposable mascara wand is poor quality and does not give you the "full effect" of the mascara. Also (this is me personally) but I would not use the mascara at the sephora stores at all. This is because even though ideally you should use the disposable mascara wand that you can get at the Sephora stores I have seen on numerous occasions people just using the wand that comes with the mascara and applying it straight to their eyelashes which to me is unsanitary. If you are willing to use a mascara that has possibly been used by someone then that is your own choice but I personally wouldn't. Another thing is that the mascara in sephora could be very old or gross after sitting there for a long time which could effect the formula. My best suggestion for you to do which I do all the time is to buy those giant mascara set samplers that sephora always have. They usually have about 10 different mini mascaras and typically are really cheap. They are also exact replicas of their bigger versions which means they come with the same mascara wand that the actually full sized versions come with. I suggest you just buy one of those and take your time trying out the different mini mascaras that you know no one else has used and comes with the right mascara wand.