1. You must read the description for accurate details about the item.
2. You can and should ask the seller questions about vital information that is missing from the descripton.
3. Before bidding, you should check the feedback score of the seller; actually read the feedback comments,
4. Make sure you understand the details of payment method and shipping before deciding to bid.
5. Beware of the special posting clailming the machine is "rare." Even if itis polished and gleaming, it may be one of the 15,000 machines madeof that model on a single day of production in Elizabeth, New Jersey.  Singer maintains a website that shows the date of manufacture of any machine if you have the serial number.  It also shows how many of that particular model were ordered made on that date. 


Then, when deciding on which machine to bid on,
1. Watch the item for a while before bidding. The progress of the bidding can be a forecast of the final price.
 2. Do not bid on items which have poor quality photograph !  Ask the seller for better picture, especially if the photo is of the back side of the machine only, and/or is too dark for seeing details.
3. Pay attention to the maximum bid feature, and make kyour maximum bid enough to cover at the last seconds. A popular machine can be "sniped" at the last 5 seconds by an auction aficianado!

About machines:
1. Many sellers do not know much about sewing machines, as they may be selling many different kinds of items.
2. Watch for items included in a listing for a machine, such as bobbin cases, power controls, wiring in poor condition, etc. A bargain may be a loss if many things have to be repaired or replaced. Many parts for old machines are very difficult to locate, such as throat plates for unusual old machines.
3. Ask if the seller has actually sewn with the machine, and if they will post a picture of the actual stitching made on that machine. This is important if the machine is purchased to actually SEW! If seller states something like: "I do not sew, but when the wheel is turned, theneedle goes up and down."  It can do so, and there could still be many things wrong with the actual sewing function.
4. If the machine has a motor, but the seller has not actually plugged it into electricity and tried it, that motor could be burned out, or need extensive repairs.  Of course, if you do not intend to use it as an electric, it could be changed to a treadle or hand crank operated machine.
5. Not all Singer sewing machines in carrying cases (without desk cabinets) are truly portable.  themodel 99 or 99K or 127, or 128 models are smaller in size than the models 15, 15-90, 15-81, 201, or 66.  But they are extremely heavy.  I see many of the 3/4 size machines with the word "featherweight" in the description. 
6. The featherweight is only the models 221, 221K, 222, or 222K machines.  They weight only about 11 pounds outside the case.  The model 301 is made like the featherweight, but weighs about 14 lbs outside the case.  It is often referred to as the"big sister of the featherweight."  Both the 221, and the 301 models are excellent machines, but beware: Many have been used constantly since they became popular, and could have worn out motors, or damaged internal gears. The way they actually sew on cloth is important.
7.  Bewary of descriptions that describe machines as "Industrial Strength, can sew leather or canvas."  Many machines were made of very sturdy, durable materials, and can do this, but were still not designed to do this kind of work over a span of time.  I wonder how many people who sew actually sew on leather or canvas with home sewing machines? 
8. I shun those postings that show items that are dirty, cords frayed and carelessly wrapped around the machine, showing little preparation for displaying. Even an old relic that is useful only for parts can be wiped down with a damp cloth and put forth in the best possible manner. Why waste a photograph and the time to post on a dirty, carelessly positioned piece of junk?

There are many other pointers for shopping for these great old machines, but time limits me.  Good shopping on eBay!