I.e., if RER ticket is purchased at CDG, can it be used in the other direction - going from Paris to the airport? (OR, if ticket is purchased in Paris, could it be used going from CDG into the city?)

Please note I'm<b> not</b> asking if they are round-trip, just if they can be used in either direction. I'm grateful if anyone knows for sure -- thank you!

yes.

Thank you, MademoiselleFifi -- I watch for these on eBay and they're usually listed as being "to" and "from."
Good to know they work regardless of direction!

Just need to watch what zones the tickets are for

Several items to consider:

1. There are tickets sold at reduced rates which require your travelling with a card showing that you are entitled to use the ticket. Some examples are tickets purchased with a: Carte Améthyste, Carte Emeraude, or Carte Rubis. Make sure the tickets you purchase on eBay are regular fare tickets.

2. The good new is that RER/Métro tickets have no expiration date and are good until used regardless of age.

3. Mobilis tickets are not good for travel from either airport.

Yes, these are important considerations. I've only purchased RER/metro tickts and have not really searched for transit passes, although I do see Paris museum passes sometimes. (Bidding on these tends to outstrip me.)

And I have had good luck with London Oyster cards -- not every traveller thinks to get that possible &pound;3 refund.

I imagine people are just selling them on eBay because they bought one and decided not to use it (the regular RER ticket). Since they don't expire, it's a way to get a few dollars for something that is easy to mail in an envelope, I guess. I don't know why someone would sell a museum pass unless they bought one and didn't use it at all. I wouldn't ever buy anything like that on Ebay as the cost is so little, what would you do if the ticket were invalid? It wouldn't be worth the trouble to complain or whatever you could do to eBay. For example, the museum pass won't be any good if it were already used once, even if a 5-day pass as it starts as soon as you first use it. that RER ticket only costs about 8 euro to begin with. Alsoo, sometimes a used ticket won't be marked that clearly in the turnstile, so it could look as if it weren't used, but wouldn't work. You couldn't buy a regular Paris transit pass online as it is only valid for certain dates, which would be over by the time you bought it and got it. Now if someone sold you an unused Paris Visite, that could be used by someone else, but again, it sounds pretty risky.

I just looked on Ebay and nobody is trying to sell any Paris RER tickets that I can see. How much do they sell for that makes it worth your time to be checking ebay for those?

These little treasures, as I consider them, aren't listed very often but I think they're great-- here's a sample item #160355324498. (2 metro tickets + 2 RER tickets, sold for $1 with a profit-earning $5.99 shipping charge.)

It's not a question of being "worth my time" because it's just another pleasant thing I do in preparation for any trip. I probably spend under five minutes a day on eBay, checking my "saved searches" for things I'm interested in.

I think you're right in that the people selling these tickets bought them and then opted for a taxi or whatever. If someone intended to scam the buyer, I doubt they'd bother with $1 items. Check their feedback ratings. And most people are kind -- one guy included "Cafe Nero" free coupons with the Oyster card I bought from him. Another seller's metro tickets arrived with a French phone card which still held value. Nice touches.

The museum passes I've seen have been, as you suggested, listed by people whose plans changed. One woman recently listed three 6-day passes for $150, explaining that a trip she planned with her mother and sister didn't come to fruition. They didn't sell and I noticed she relisted them individually, $50 each and they sold immediately. The savings (over the regular &euro;60+) wasn't that outstanding, but someone thought it was worthwhile making the deal.