I will try my best to address your questions, but please note that I do not have personal experience on both sites so my answers aare more or less based on the information that I have gathered online -
1.Is there a time limit for each listing, if so how long ? and 2.Do the sellers get alerts before their listing ends, if so what are they required to do ?
Alibaba.com - Not that I know of and I have not found any information to answer this as well. But the way I see it, there might be no deadlines since companies that post on this site are manufacturers and not direct resellers.
To get more answers, I decided to open an account on Alibaba.com and found that you can list up to 50 listings on your free account. All your listings have to be approved by them before it goes live. The approval period usually takes 24 hours.
DOTmed.com - Your listing will remain on DOTmed for 35 days. You will be notified prior to 35 days that your listing is about to expire. If you do nothing the listing will be deleted, but if you follow the instructions in the email the listing will remain for another 35 days.
3.What's the process for sellers that have large inventory, how these sites verify that the data is updated and that the products actually exists. for example :
I am a seller with an inventory of 1000 products :
*Is there a special upload tool for wholesale product (like eBay Selling manager pro)?
*Is there any special form that the seller receives which requires him to check which products are still available and which aren't ?
Alibaba.com - They offer a selling tool called "Batch Upload Form" to help you upload several listings at one time. To learn more about this tool -
Link hidden: Login to view
DOTmed.com - Sorry, I was not able to find any tool similar to eBay's listing manager. You must remember that there is a limit to the number of listings you are allowed to make plus you can only post an equipment or an item for sale if it is ACTUALLY owned by you.
I recommend reading their Listing Legal Notice and DOTmed's Compliance and Compliance Complaint Procedure for more do's and don't's.
You might also find their HELP page quite helpful - Link hidden: Login to view
Hope this helps, somehow. All the best!