Baseball postcards are pretty much a popular subset of postcards in general. You can sometimes find them at antique shops in the postcard box for $1. It's usually more since sports are a popular topic.
Even with mainstream postcards there aren't many checklists. Mostly because the main companies used the same images over several years, and the postcard could have been "published" by the main company, or by multiple local businesses. A few years ago I saw the collection of a local collector who collected Lowell, Ma postcards. He had between 5-600 of them all different. And he didn't collect different backs. Nearly all of them were prewar.
The biggest producer was Curt-Teich. Their archive is online
http://www.lcfpd.org/teich_archives/...tion=home.view
Yes, that's 365,000 different postcards they show.
And just from one manufacturer. Detroit publishing was also pretty big.
Add to that sort of thing the probability that some from small towns were made locally.
And Real photo cards could be made from any photo negative by several companies including Kodak who offered postcards as an option.
Postcard collectors don't get as hung up on condition as we do. They have essentially the same grading system that was used for baseball cards years ago.
mint- not mailed and no wear.
near mint- not mailed only very slight wear
excellent- slight wear
vg
g
junk - The whole range of wear.
And with most dealers the condition won't matter much for pricing a common card. They just put them in the dollar box.
Mailed cards present a different range of challenges.
My brother collects West Point postcards. But has become less interested in unmailed ones or ones with the usual "saw this today" sort of message. The few he has with messages from Cadets are much more interesting. Just like a baseball postcard with a message from a player or about a game would be more interesting.
It's still a hobby that hasn't gotten too caught up with grading or completeness or anything like that. And although there are some expensive postcards- I know of one by Mucha that brought around $10K - there aren't many fakes. There are reprints of some really great images, but they're usually obvious.
Steve B