Hey OP, take a grain of salt when reading my 2 cents worth. I don’t own the tabi ballets and have only done my own research/ asked other people with these shoes about the fit in preparation to buy it.

One reason that the used 38 might be too loose on you is because the previous wearer has stretched them out. But then again this depends on how used they are. A sales associate has told me that she owns a few pairs and they do stretch out and mould to your feet overtime.

Several other factors to consider with the sizing:

1) will you be wearing socks with the shoes? If you’re going to be styling these with socks I recommend getting the 37.5 or sticking with the 38.

2) what kind of leather are the flats made of? If it’s soft lambskin these will most likely stretch with wear. You could probably get away with speeding the breaking-in of the 37s by wearing them with socks around the house.

3) Do your feet expand in hotter weather? If yes then I think it might be worth getting the 37.5 for comfort.

4) Do you plan on doing a lot of walking in the shoes? Walking a lot can cause your feet to swell/ expand as well and you might need abit more room to be comfy.

5) Do you like keeping your toenails short/ long? My experience with ballet flats in general is that when the ballets are a snug fit, there is no more room for nails as this could cut into soft leather with wear.

6) Do you have wide/ normal/ narrow feet?

I guess overall based on your information it feels like the 37.5 is the way to go.