OK, so you have this notion that maybe you'll publish your game in PDF format at sell it somewhere. If you just want to raise product visibility, do it. If you are planning on making really substantive sales, think again. Most PDF games sell under a hundred copies, and I'm guessing, that most PDF board and card games sell fewer copies. Possible exceptions probably include the games BTRC does, as Greg Porter has a rabid fan base that buys what he does, including PDF board and card games.
There are a variety of works out there on PDF publishing of games, specifically the E-Publisher series (One Book Shelf, Inc.) and the E-Publishing 101 series (Adamant & Ronin Arts). Check the product pitches and reviews for details. A lot of these books are targeted at RPGs in PDF form, but they'll have information of interest to the PDF card and board game publishers too.
I think that, overall, two words sum up why PDF board and card games don't sell: "James Ernest", or alternately "Cheapass Games". People would rather pay $4 or $7.50 for a fully printed, cut, and stored Cheapass Game than they want to spend $3.00 to $5.00 on your games, then buy heavy card stock, print them out, cut them out, get a bag to store them in, etc.
Neo Productions and BTRC are they only two companies trying to sell PDF versions of customizable card games at this time. Neo Productions is doing it for their Final Twilight game, which is available in physical form and available for play via the Gatling Engine on CCG Workshop. So, at some level, Neo Productions is raising awareness of their print game (available in distribution channels). BTRC has taken a different route with their City of Heroes CCG. They have the most amazing cross between Neo Productions and BTRC's ideas to date. Not only do they have a browsing script to let you look at previews of cards, not only do they have fully playable decks in PDF format, but they have a way to build your own character cards for the game, online, for free! They are doing this to try to hook players of the COH MMORPG on the game, by having them take screen snapshots of their MMORPG characters and move them onto cards for the physical CCG. They are also coming up with insidious cross promotion schemes (brilliant) to give people physical cards for playing the online game, and access to secret special features in the MMORPG for playing in tournaments with physical cards. AEG has the most brilliant cross promotion of a game that I have ever seen.
Another physical CCG that's promoting online is doing something I have never seen -- they have put up scans of all of their cards online: NECA's "Nightmare Before Christmas Trading Card Game". Some people have expressed concern in the RPG industry about how giving away free copies of your rules will cut into sales of physical products. I think that for card games, this is probably not a huge concern. I could be wrong. But I expect that the desire for high quality physical components will be really high for people playing such games more than once or twice, and that people will invest in the physical product if they like the game enough. Only time will tell if I'm correct.
Another avenue of electronic production is the "play online route" (which Neo Productions is also leveraging). Playing online is an absolute must for niche market game players looking for people interested in the same game.
For private play between friends, there is no better engine that I've found online for playing card games (customizable or otherwise), online than Lackey CCG for Windows. It is an extremely customizable engine that can be adapted to almost any card game. Its biggest problem is that you have to give out your computer's IP address to people you want to play with, making it undesirable for play with people who you don't trust. It's more robust than most card engines for services that charge. It is currently available for free in beta, but will probably cost $20.00 when it's ready for roll out.
If you are interested in designing for online play of a board game instead of a card game, then the Vassal Engine or Game Maker may be just what the doctor ordered. These have the limitations of Lackey CCG, in that they require you to give out your IP address to another player.
Game Table Online licenses games for online play.
CCG Workshop is another major place for online card games. It is a pay service that lets people login and find opponents. It has a wide variety of games. I'm actually surprised this place wasn't shut down by a lawsuit years ago, because while many of their games are licensed, they have copied dozens of out-of-print card games (and some in print card games), many without a license (according to their own site info), and charge a fee (for some games) for playing against opponents. That caveat in place, if you license your game to them, you can expect some added visibility from this growing community of card players, who are keeping even out-of-print games from going totally dead.
Most of these solutions share a common problem which keeps some vendors from getting too interested in them: you have to make at least your card art, and in some cases images of entire cards, available to every player who wants to play. This will make some people quite nervous if they have a physical card game and are afraid of assisting people in pirating cards. Most of the cards are low enough resolution that this isn't a real problem, to be honest.
The other route people with a bigger budget can take is the route that games like Chron X and Star Chamber have taken, which is to develop a real CCG environment where people buy electronic decks and packs of cards and have secure servers to trade these virtual cards. Wizards of the Coast does this with Magic Online as well. I'm surprised that people will pay 2/3 to full price for electronic copies of print cards, but they do.
Regardless which method you choose to publish your game online, I strongly recommend that read the article on Software for Card Game Design. You'll definitely want to maintain your cards in some easily alterable format so that as your cards need errata (and for a CCG, at least, they probably will eventually), you can make the modifications.
If you decide to sell your game online as a PDF, there are a few places you should consider contacting: