Oh, one more thing I forgot to mention is that we also pass on the fixed fee we are required to pay when we send the cryptocurrency you are buying with your Steem to you (similar to a bank wire fee). So if you are buying a small amount of bitcoin, the fixed fee we have to charge you to cover our costs for sending can affect your price. In such cases, it's better to buy a somewhat larger quantity so that the fixed fee has less affect on the overall price you get. Also, different coins have different fees, so if your ultimate goal is to get to dollars or euros, you can save money by choosing an intermediate coin with cheap transfer fees. Here's an article with more details about this: https://steemit.com/blocktrades/@blocktrades/how-to-convert-steem-sbd-to-fiat-more-efficiently