I have a greeing card business (not handmade -I'm not artistic enough) and I come across lots of handmade cards in my travels -most fairs will have a least 2 people with them and there are numerous websites.
Handmade cards are fantastic for a special occasion and I do buy them. The only problem from your point of view is that I am absolutely positive there is no way the maker can be making a profit. The main reason for this is that so many people do it as a hobby and decide to sell their excess stock to buy more materials -so they tend to set the retail price. But they don't take account of their time, travel or even half their direct expenses.So you can often buy a handmade card for less than £2 that would have cost more than that to make(if a lot of embellishments are included).THis is a real shame, especially as Clintons etc. now sell what they call handfinished cards as well -but they charge £3-£4 for them.
There are a couple of ways you can make money at this -one, as mentioned already is the wedding/announcements/special stationery area -where prices remain very high. The other, I've seen a couple of people doing is to sell some of the equipment and material alongside your cards -that way you will recoup some money from the hobby brigade.
Is your girlfriend an artist. The company I work with are always looking for new artists for their cards -and if your designs are chosen, that can be very lucrative.
If she would like to chat some more about the card market in general, please feel free to contact me
Anne