Basically AC3 is the old name for 'Dolby Digital'. Simply put, the old AC3 (DD) signal from the LD player is not the same as the signal that comes out of a modern DVD player, although it is still DD 5.1. The signal that comes out from the LD player requires a box to 'convert' it into the modern Dolby Digital format that we use today. This will enable your old LD player to hook up to a modern surround amp or even a soundcard on your PC if it has the right SPDIF connector. This 'converter' box is called an 'AC3 RF demodulator' and these units are quite often available on EBay. The AC3(DD) signal is still 5.1 and does not get down converted to 2 channel.
A lot of older consumer level LD players and most (all?) industrial LD players including the V800/V8000/V4300D do not have the AC3 RF output plug and consequently do not give Dolby Digital. Most LD players including the industrial players can be modified to give AC3 output if required though. There used to be quite a few companies advertising on the interweb who did these mods and there are still a few of the circuit diagrams floating around. If you did decide to get an industrial LD player, then chances are that it can be modified to give AC3/DD 5.1. It might actually work out cheaper and easier though to simply buy another cheaper consumer level player which already has the AC3 RF jack plug and grab the sound from that to sync up to a decent video capture from an industrial player like the V8000. ie: use the industrial player to capture the better picture and use the consumer level player to grab the sound.
Just to confuse things a bit more, LD also had available a 16bit digital PCM 2 channel audio track which is the same quality as CD. This is primarily used on the majority of LD discs that are available. Simply put, AC3 (DD) came out of the AC3 RF jack plug and the PCM audio came out of the 'digital optical audio' connector. When they mentioned the ‘digital audio output’ in that online auction, they were talking about the 2 channel PCM audio. If you were to buy that player, then you would have to modify it to get AC3 Dolby Digital.
Don't worry about the BNC sync plugs, you aren't likely to need or use them.
Hope this helps.