Well, I think once you try the SG1 you'll see what the fuss is all about. The SM3 is nice, but the segment shift on the SG1 will make you realize how much clunkier the carriage shift on the SM3 is. And check out all the characters the additional keys give you, to say nothing of the paper injector.

These are hard to find -- $125 seems a lot, but a bird in the hand.... If it seems to be working OK, you should go for it. And then we will be very interested to hear your impressions!

Yes, location is key to value.  Here in Australia, for instance, I should be content to pay 125 AUD for an SG1 in full working condition, and think it a reasonably price, though not 'a bargain'.

In Spain I´ve never ever seen a SG1, so the price could range for a pittance (not probable but you can always find a bargain) to outrageous. As far as I have "suffered", overpriced typewriters are the ones that are already on the market. I mean, if everyone is selling Underwood 5´s at a 3-figure price, you´ll probably do the same with yours. Inertia, maybe?

But if somehow you have something "uncommon" like a SG1, maybe you just ask yourself how much do you want to get for it. Maybe you just want other person to enjoy it, or maybe you want to get richer than Bill Gates in one go. Thing is you don´t have a lot of overprized examples floating around. I mean, how the Underwood 5 can be SO overprices here if it´s fairly common? Same Goes for the Corona 3, and to some extent for the Hispano Olivetti M40.

In addition, I should mention I DO WANT a SG1, and so far the nearest I can find are in Germany... If normal shipping gives me nightmares, I don´t want to think about international shipping

Javi wrote:

...how the Underwood 5 can be SO overprices here if it´s fairly common?

The Underwood 5 (it's just as often a 3, 4, or 6) is not in the same category as a SG1 because it transcends the interests of typewriter enthusiasts and collectors and is also bought by those who don't have any interest in typewriters beyond having one as a decor item. To their uneducated eyes the Underwood 5 is the iconic antique typewriter. It has shiny keys, you can see inside of the machine, and its covered in old-looking paint. As a result there's a greater demand for early Underwoods, unlike the SG1 that usually only draws the attention of typewriter aficionados and people who want a robust, high-performance writing tool. 

There are so many of those early Underwoods in my area that it sometimes seems like they're still being made, and typically they are quite expensive. Those who know better just laugh at the ridiculous prices and wait for a $25 one to turn up, because they always do, and we leave the $250 ones in antique shops to those who need a fancy bookend for their library or a fancy paperweight for their desk.