Hi everyone, here’s my cliché ‘long-time lurker, first-time poster’ first post.
In real-life, I am having to take care of an aging parent and it is difficult to keep a regular job. So I have an eBay store and I would like to try and promote it here.
What I would like to do is advertise one listing from my store once a month and not bump it further (except to answer questions).
If this is all good, should I list each monthly listing in its own thread or just run one ever-growing thread?
Thanks for y’all’s help! If this idea can work, I will subscribe and get to it. Otherwise, I will still hang around as a freeloading gadfly. 
Only those who are paying subscribers have posting privileges in the Marketplace forum. It’s one of the benefits of being a paying member. Guests cannot start threads in the Marketplace forum. The first thing you would need to do is purchase a membership subscription (click on UserCP for your control panel on the upper left, thebn Paid Subscriptions down on the lower left of the control panel, and follow the on-screen prompts from there).
Once you have purchased a membership subscription, then yes, you can advertise listings from your ebay store, or any other store that you might have. The only real restriction is that it has to be your store. You can’t advertise someone else’s store for them. Since in this case it is your store, it’s perfectly fine.
You may also link to your Marketplace threads in your Signature. Just link to the current or most relevant Marketplace thread. We don’t want to see a signature with a year’s worth of Marketplace links on it as that would be a bit obnoxious.
The Marketplace is the only forum where image posting is enabled, so as long as it’s in a Marketplace thread that you started, feel free to post images relevant to what you are selling there.
I don’t see an issue with either a monthly listing or keeping it in one thread. Either seems fine to me.
Did somebody call my name? 
Marketplace services are only available for paying members of the board; it’s not an option open to Guests. Guests can reply in threads in Marketplace but not start threads or offer things for sale themselves.
Becoming a Member is easy and convenient. It’s also $20 a year for you, or $35.00 for two years. You support the Dope and all your friends here and you do a little something for yourself too.
Click on UserCP at the top of the page on the left hand side, then Paid Subscriptions. Follow the prompts. The Straight Dope takes most credit/debit cards, PayPal, and now Venmo. There could well be other pay services that work in the subscription window, you’ll just have to see what your choices are when you get there.
Don’t be confused by the fact that your transaction is being processed by PayPal. You do not necessarily need a PayPal account to make this transaction, though you can certainly use it if you want to.
As a Member you are entitled to start threads in the Marketplace forum and offer things for sale, as often as you like. You may offer items directly for sale; you may list them on ebay or other sites and link to the Marketplace. You are not confined to once a month, you may list items and sales within reason, of course, and I’m sure you are a reasonable person.
The complete rules are here: Marketplace Rules
Hope this helps. And good luck!
Jenny
your humble TubaDiva
Administrator
Before you lecture Marginalia too much about how guests can’t sell things through Marketplace, note that the OP clearly indicates that s/he he understands this already.
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I see ECG was ahead of me – probably not for the first time today --sorry 'bout that.
Jenny
your humble TubaDiva
It wasn’t intended as a lecture, and my apologies if it came across as such. Just wanted to make sure we were all on the same page here, and wanted to make sure that everything was clear for someone else who happened to come across this thread and might not have picked up on the significance of the OP’s “will subscribe”.
My thanks to all of you for your thoughtful and detailed replies, well above the call of duty. I hope this will help put a little extra coin in the family coffer from now on. Y’all have done some good. And if some facts were already known by yours truly, this thread may help others not so informed (I love simple information, the only resource that can be shared infinitely).
It will be my pleasure to subscribe shortly and help out my favorite board with a little coin of my own. Thank you again! 