I wear a 7 1/4 baseball caps.What would fit me on this hat?

I wear a 7 3/4 and the xxl hat fits me well. Not sure if that helps at all.

Seems from the reviews that the hat arrives crushed even if the box is uncrushed. Is the seller Peter Grimm or a 3rd party?

It was a third party, but mine came very well protected and in perfect condition. It’s a great looking fedora! I get compliments on it all the time! You will not regret buying it!

why are there two size s/m and two size l/xl? whats the difference

One is bigger. ha ha. no really you need to find your hat size, consult a sizing chart and determine which hat is for you. I only saw s,m,l, & xl. hope that helps.

Will the hat get speckled or stained if it gets wet or rained on? Any issues with staining from sweat?

I'm not sure about speckled or stained, but straw hats should not get wet if it can be avoided as it will cause the material to warp and bend. Do not wear it in the rain or in the pool. Perspiration shouldn't stain the actual straw, but it might stain the decorative band round the hat. It's a good idea to wipe out the inner band (the one inside the hat) after a day of wearing it. Even better, consider carrying a handkerchief on a hot day so you can occasionally wipe off your forehead and give the inner band a quick wipe down.