I wrote a bit of a rant about this on an ad, because dishonesty is one of humanity's major problems.
It's a lot more common in the UK than here, but when you see the words "Industrial Strength" in a sewing machine ad, together and in that order, I can say without any doubt or hesitation that the seller is trying to rip you off.
The seller is attempting to make potential buyers believe that old machines like theirs are in some way just like industrial machines. This is a lie. There is no such thing as industrial strength and a machine is industrial or it is domestic. Actual industrial sewing machines are many times heavier and many times more expensive than domestic machines. People who own factories aren't so stupid as to buy a $3,000 Singer 132K6 to sew upholstery if a $90 Singer 201 plus cheap walking foot will do just as well. If you believe the 201 would do the job, you're dreaming. Industrial machines are made for a very specific task and are made to run at high speed, non-stop for half a day, every day. Your domestic machine is not.
I'll point to an article from a fellow blogger. He describes all the differences between industrial and domestic machines and explains all the reasons they aren't the same.
The other thing not to be very suspicious of is "Semi-industrial". It's true that Singer made light industrial machines for home manufacturing (e.g. 103K, 206K10) but these machines are always mounted on benches with large treadles or bench mounted (and very large and heavy) universal motors.
On eBay UK last week I saw a 99K being peddled as an industrial strength machine! These aren't even full size and have quite a small motor attached. However, there's no way to complain or report the dishonest seller for lying or misrepresenting an item. I doubt this is an oversight for eBay.
Heavy doesn't mean heavy duty: It pays to be suspicious if anyone uses the word industrial for a domestic sewing machine.
Do some research. I'll make it easy for you.
Assuming you have an old Singer, here are two invaluable resources:
ISMACS model list
ISMACS Serial numbers
Look your machine up. The serial number will give you the date of allocation of the serial number of your machine, which will be approximately when it was made. It will also tell you the model number.
The model list gives you a bit more information about what your machine is, the needles it takes, and other information. The usage column clearly tells you if the machine is domestic or industrial.
ISMACS is the International Sewing MAchine Collectors Society. They've collected a lot of information that can help you in making a good choice. You can always ask me or any other enthusiast. Not ripping people off is one of my favourite things.
I don't want you thinking that the 201 isn't an awesome domestic sewing machine either though. If you sew at home you can't do better than one of these little wonders. My favourite is the 201K23. I like the look of black ones the best but the tan versions seem to keep popping up in better condition.