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public buying and selling,....."

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I am of the impression that although most Trads would agree that we should try to refrain from unnecessary buying at department stores on Sunday many of them find nothing wrong in buying tickets to entertainment events or to buying a meal on Sunday when it is not necessary.  It seems to me that by doing the latter we feed into the secular culture (which profanes Sundays in every way imaginable) by treating Sundays like any other day of the week.

Public buying and selling is generally not permitted ON ITS OWN.  But it's permitted when done in support of NECESSARY activity.  I can purchase medicine at a drug store.  I can purchase a meal at a restaurant.

Yes, the logic for entertainment is a little bit weaker.  Eating is NECESSARY.  Entertainment is PERMITTED.  Labor is licit in support of necessary activity.  Is labor licit in support of PERMITTED activity?  To me, since the entertainment isn't necessary, by extension the work done in support of it is likewise unnecessary.

It's like the one theologian I read who said that it's OK to drink beer between meals on fast days ... because it's a liquid.  But, then, he says, it's OK to munch on some snacks with it ne potus noceat, so that the food doesn't cause harm (e.g. upset your stomach ... due to drinking on an empty stomach).  That's such a huge stretch.  My brother (also a seminarian) and I used to joke that I could go drink a beer and then eat a Big Mac with it ne potus noceat.  We'd toss that phrase around as a joke ... rather mocking this particular theological conclusion.

Another theologian said that something was considered drink (vs. food) if you could get it through a straw.  So my brother and I joked about blending up solid food and sucking it through a very fat straw.

I once also read a particular theologian who used tortured logic to declare that if a priest did not offer Mass on Sundays he was not required to ATTEND Mass to meet his obligation to keep the day holy.  That's because the Church law requiring Mass attendance was, technically speaking, addressed to "the faithful" and not to priests.

Jesuits were notorious for this kind of reasoning.  There's actually a term for it ... "Jesuitical logic" or "Jesuitical argument".  In doing so, they completely undermined the spirit of the law.  Not only that, they made Catholic moral theology appear very Pharisaical to the world.  I think they did it just to show how clever they were.

...what about sewing clothes?  That is menial labor, yes, but...if I find it's a form of recreation, is it a sin?  It's not as physical as yard work, which I consider work to be avoided on Sunday.  And yet there are those that find yard work relaxing, and not labor intensive--a form of recreation.   Any thoughts on this?

Are you sewing for money? Then save it for a weekday.
If you sew because you have a hobby, then go ahead.
If you're sewing because you need a new dress, and it doesn't take away family time, OK.
It's just as physical as yard work, though it doesn't need as much physical energy.
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I garden whenever the weather is good for it - not too hot nor too cold nor too wet.
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As long as Our Lord is given priority - pray always - and unless it is unnecessary servile work, go ahead.

My wife sews on Sunday for RELAXATION.  She finds the focus and concentration required to be a great comfort and relief from all the noise and stress she endures during the week.  For her, it's little different than working on an art project.  So if it's for fun or relaxation, there's no issue there whatsoever.  Nadir's answer was right on the mark.

Maybe we should keep in mind what the purpose of Sunday is in order to make a good judgment.  Principally we are worshipping God and drawing our minds to the things of God and of Heaven.  Whatever aids us in that regard and which has not been prohibited by the Church is acceptable behavior for Sunday.  I can think of a few things that may not be explicitly prohibited but which I think would probably violate the spirit of the law.  Such as violent sports, crafts or sports which involve loud machines, strenuous labor even if subjectively it is relaxing.  Strictly speaking these things may not be sinful in every case but it seems that it should not be too difficult to find some other activity which is more in keeping with the spirit of the day.  We are not all called to be monks but we are called to be holy.