Aside from everything else mentioned, I would look for a seller who clearly states in the ad that they are very experienced in packaging sewing machines. They should describe how they pack too.
If you don't see this information up front (which is the best way to see it, as it indicates the seller realizes that packaging is critical for sewing machines), email the seller before bidding and ask how the machine will be packaged. They should reply with a detailed description that includes double-boxing.
The key to safely shipping sewing machines is to pack them so *nothing* inside moves during transit, and so that the case is protected against knocks with a thick padding of styrofoam or other material. Packaging a sewing machine properly is something of a science. Occasional sellers of machines usually do not understand how to package properly, and many of the packaging places also do not understand how to do it properly. A seller who takes it to a packaging place still often won't get the correct packaging.
Always take out insurance on a sewing machine purchase. Whether or not the machine has been packaged properly, you need that protection against damage in transit. Understand that you are paying for the insurance so, if there is damage, the machine is still yours, you have to file the damage claim, and the damage amount will be paid to you. This is not the seller's responsibility!