There are advantages and disadvantages to each. A dealer with a regular storefront will likely have a great selection of instruments to choose from, both new and used, and will give a warranty. However, dealers often have the high overhead that comes from needing to be in a prime location, keeping the store staffed, keeping many pianos in tune, etc., so prices will usually be higher. The attributes of a good piano dealer are discussed here.
Some piano tuner/technicians also sell used or restored pianos from their home or workshop. Their selection is usually limited to a handful of instruments, but their overhead is usually lower than a regular dealer’s, so their prices may be a little lower. Some also sell new pianos, but if they do, they may also be incurring the same overhead costs as a non-technician dealer, so their prices may be similar, too.
The major advantage to buying directly from a private party is that there is no overhead or profit margin built into the sale price, thus a greater opportunity to get a good deal. However, purchasing from a private party (usually the piano owner) also carries the highest risk. There is no recourse after purchase, so if there is a problem, you are on your own. There is also no selection of multiple instruments to choose from (you have to shop at multiple locations), no selection of new pianos at all, and you have no assistance in moving or servicing the piano after you've made the purchase. We highly recommend that, before buying from a private party, you have any piano you are considering evaluated by a piano technician. Major defects can be concealed by the seller, or may be unknown to him or her, and the piano can be unplayable in a short time, or be expensive to repair.
For more information about this topic, see the article “Buying a Used or Restored Piano.”