They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.
And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.
Holy carp dude! Do you have a warehouse full of these things that you're selling? Your wild guesses are no better than anyone elses here.
First, I'll admit that my estimation of the ampacity of the 24awg winding was off. I based it on a transformer winding rule-of-thumb of 2.3A/mm^2 and then doubling it because it's a variac. I looked at two variacs of mine, both Staco, one with 24AWG rated at 3A and one with 26AWG rated at 2.25A. So reputable manufacturers are using about 6.0A/mm^2.
However, I don't see how you justify your assertion that they tell you exactly that the maximum current is 2-3 amps (as if "2-3 amps" is exact in the first place) especially since it says 500VA on the front as well. I would argue that that labeling would permit you to connect a 5A load at 100V and let it roast.
Lastly, how much obvious QC will you ignore in your support of this junk. Poor solder, wiring internally wrong, not grounded, etc, and you think you can just fix all this (on a brand new unit) and just assume that all of the other things that you can't test or didn't notice will be just fine? That the enamel insulation is top quality, the iron core is sufficient, etc etc? When a product or company has severe, obvious issues the first thing I think is "what else did they screw up" not "gee, I can patch that up and I'll have a bargain". If I'm going to patch something up, I'd prefer a high quality product that has withstood the test of time.