I produce my own to sell myself (on my market stall and website) and have tried to work the numbers out for supplying them on a more "wholesale" basis and it is really hard to make the maths work out without putting in a big investment of time and cash.
Most small gift/card shops work on a 240% mark-up (a handy nugget I learnt from a friend who runs a successful card and gift shop) so for them to sell a greetings card at £2 they aren't going to want to pay more than 83p. Out of that 83p you have to take the photo, design the card, get it printed, purchase an envelope, purchase a cellophane sleeve and then fold and pack the card. If you can do the photography, design, and packing yourself and order a fair chunk of cards you can just about make it work and maybe make 50p on each sale (if you don't pay yourself for the time of doing the design, packing etc....and packing takes forever!).
You then have the fun of joining the competition and tyring to get shops to swap to your cards, as most already have someone. If you have some stunning local photos then you might stand a chance (for some reason the really big guys use really mediocre photos).
The comes how to sell them, do you do mail order? do you create a website? do you have minimum orders? how to keep track of stock? how do you deliver them to the customer (if they are all local you can save a fortune and deliver them yourself, but that is more time). Do you register for VAT? Do you take payment on order or do you issue an invoice? Then add time at the end of the year for sorting out all the paperwork.
After all that you have to allow the designs that just don't sell. I have some cards I really like that just have never done well so I have about 100 of them just sat on a shelf, so they are prett much just a straight loss.
During all this you need time to go out and get more photos so you can update your range, but then you need to have sold enough cards to warrant another order from the printers because you have to do another large run to keep the price down, so you have to balance adding new designs and ordering more of what sells well. You then also get cards that sell well once and then for some reason stop being so popular so you can't just bulk load on those in case they end up on a shelf.
None of this is different to any retail really, but you are just the one taking the risks and working on tiny margins. If you have a good product that you can shift a lot of and can get for the right price then sure it will work......but in the current climate nothing is guaranteed and I really don't know how many small shops are survivng at the moment.
I learn the other say that Salmon (who were massive and had reps visiting card shops etc) have stopped publishing anything and are now just licensing their images -
http://www.jsalmon.com/ - I'm not honesly sure if this is good or bad news. It obviously means it is working on a massive scale, but maybe it opens the doors for people like us doing it on a smaller scale.