I've bought and sold things on ebay and so far everything turned out O.K. Although as you already mentioned a pic is important. Remember "A pic is worth a thousand words". You can find a real cheap digital camera nowadays for anywhere between $20-50 it won't be a "top of the line" camera, but it'll do the job. You could even use those $5-$10 cameras you connect to the computer for chatting.
I've also bought things on this forum and have had good and bad experiences. Don't run it like an auction, that'll turn everyone off. Decide what you would be willing to take and give it to the first person who replies and sends you the money. If the first person who replied doesn't send the money in 1 week, then sell it to the 2nd person who responded. Most of all be HONEST about the part. If something is broken or defective, then say so. That way the buyer will not be disappointed and you won't have any headache about any complaints.
BTW what kind of W124 86' 300E parts do you have?
Email me
jrmd01@yahoo.com