I am trying to make a reservation online at the Trenitalia website for Venice to Como for the end of May (this year). This route does not allow me to purchase online (there is a 'No' where I usually have seen the 'Buy' option). Does this mean that my choice is sold out, or are some tickets just not offered as an online purchase? Thanks!

Some trains - chiefly the Interregionali - cannot be reserved on line.

Eloise is correct. When we did this route last year, we bought our Venice to Milan tickets online. But we had to wait until we got to Venice to buy the tickets from Milan to Como.

I tried to buy the Venice to Milan route and that does not offer the 'Buy' option either.

Are you within 60 days of when you'd use the tickets? If not, then you can only look at the schedules, but not buy yet.

Very strange. I get the same thing: you can buy the IC (InterCity) but not the ES (EuroStar).

But I tried it on another date at the end of May - May 28 - and the ES are available. (I don't remember what the first day I tried was.)

So maybe the Eurostars you are trying to book are, in fact, sold out.

If you have to travel on a certain day but the ES is not available, take the IC, but make a seat reservation. It's an extra cost (not much); it is not inluded in the fare as it is for the ES.