Starting from the 6.1.1. version for Magento v 1.x and all versions from Magento v 2.x M2E Pro uses it's own Cron Service, so there is no need to set any additional settings for it.
Cron Service is a predefined free service from M2E Pro. The principle of its work is based on the regular requests to a particular endpoint in Extension with a further run of synchronization tasks at the moment of this connection.
So M2E Pro Cron sends the request each minute to the endpoint in your Extension. However, in some cases, these requests do not receive the answer. Thus the synchronization cannot be run. So when you go to the Help section of any of M2E Pro tabs (e.g. Sell on eBay > Help) you can see the line ‘Last Run’ colored in red.
So what can cause such situation and what to do?
The situation is mainly caused by blocking of outgoing connection - the M2E Pro Servers (as of February 01, 2018 such servers are running s1.m2epro.com and s2.m2epro.com) and Cron Service (cron1.m2epro.com and cron2.m2epro.com) must be in a list of allowed incoming/outgoing connections. So you should check if it is not blocked by your Firewall or by any other settings of your server.
Are there any alternatives?
Magento Cron can be an alternative solution when M2E Pro Cron fails. In this case, automatic synchronization will be performed based on the Magento Cron settings.
Check the instruction for Magento Cron set-up.