Not sure if anyone noticed but if you travel to Hunter's Dream your Blood Vials and Quicksilver Bullets are replenished (partially in the case of blood vials, dependent on how many were used). You can in turn sell them to the messengers for 800 blood echoes with respect to 20 blood vials and 600 blood echoes for 20 bullets. Warp to another location and warp back to Hunter's Dream and you'll have 12 blood vials and 20 bullets given to you. This is what I've noticed anecdotally, so hopefully this helps people in the beginning stages of the game where this is good number of echoes or if you just need a few more to purchase an item. For those later into the game, this is a negligible number or for the impatient this won't be efficient because of the games loading times. Hopefully this helps someone. Apologies in advance if this was already mentioned.