I won't say the seller is blowing smoke, but what he's blowing isn't completely fresh air. His antenna may do exactly what he claims, but with a catch or two. The first thing I can think of is that I hope you have a tuner.
Any half wave double/dipole can probably be made to work on an odd harmonic of it's design frequency, not with a low SWR, but probably a usable one. How that antenna will radiate, it's radiation pattern get's unpredictable if it's not used on it's design frequency. It may do what you want, b ut not as well as an antenna resonant on the 'higher' band. Then you get to factor in all those other thingys, like heigh, what's around it, etc, etc. It's OCF, off center fed, so will not have the same SWR as a center fed dipole. Those OCF antennas do work. They may work 'better' away from it's design frequency than a center fed dipole. But don't bet the farm on that till you try it, there are just too many variable in mounting, etc.
Hate to tell you, but that 'original Windom' isn't a Windom at all, it's an OCF antenna. The original Windom antenna used a single wire feed line and a very good RF ground system. That single wire feed line was part of the radiating antenna, basically a sort of short vertical antenna with a large 'top-hat'. That good RF ground is an absolute necessity. Figure on ground radials, not ground rods! Ground rods make for a terrible RF ground no matter what you're told. Good safety grounds, but not RF grounds.
What you are looking for is a good -compromise- antenna. Just keep in mind that it's a compromise in some ways, ain't never gonna be -good-, just 'usable'. When doing/using something like that it's a very good idea to use a tuner along with it. A tuner won't 'cure' that compromise but it will make it more 'livable', easier to use. A simple dipole on the lowest band to be used and a harmonic on other bands is probably about as simple/cheap as it get's. Get it as high as possible/practical and that's probably going to as good as anything. Between two trees is nice, but over a tree or two will work too. It touches those trees? Okay, so compensate a bit, use a tuner to get it into a usable impedance range and don't worry about it. It won't 'burn up' your radio.
There are no 'miracle' antennas that do just as well as a classic 'good' antenna that's made correctly and mounted decently. If by chance you find an antenna that works well for you in whatever situation you're in, that's the one to have. Change that 'situation' and that 'best' may not be so good anymore.
Good luck and have fun.
- 'Doc
:blushing: