As for saleability check eBay but in general most jerseys are unsaleable except for the team-issued game jersey and half the things from Mitchell & Ness because they re-create game jerseys no longer in use. I have a Mitchell & Ness 1975 Yaz like this,
This is all my experience. I'm sure other thrifters have different paths to turning a buck or two with sports jerseys & have things to add to this.
I've had good luck with jerseys, actually, but you have to know what you're doing (I do not) and I've found that the market can be fickle. Case in point, I flipped a Kevin Garnett T-Wolves jersey that was, at least to my eye, nothing special--nylon mesh, not sewn on--for $27 last year then could not unload another one just a few weeks later. I've never had trouble flipping Dennis Rodman jerseys, and a Jerome Bettis sewn-on from his days with the Rams got snapped up instantly with a BIN of something like $50, and I had numerous other inquiries on it. OTOH, I can't unload a Vladimir Guerrero (Angels). I stockpiled a few STL Pujols jerseys that were cheap and plentiful after he went to California, but I'm not sure I'll make out, given what a flop he has been there--my thinking was, he would have such a monster career that they would be worth something in a few years. OTOH, I'm wondering whether I made the right call in passing up alll the Cleveland LeBron James jerseys that were out there not so long ago. Now is the time, I suspect, to peddle my NWT Clippers jersey by Hardwood Classics. Other jerseys I've flipped without trouble are Tom Brady, Michael Jordan and a Bulls jersey from some guy who went to college in Iowa whose name I can't recall--there was actually a fair amount of interest in that one.
I love flipping jerseys because you don't have to measure them, they're cheap, easy to mail and you can get ten times or more what you've paid if you pick right. But if you pick wrong, they won't move at all. Now that I have a smartphone, I hope to do a better job of figuring out which ones to buy.