First, the OEM versions can only be sold with Corel authorized hardware, e.g. buying an OEM version separately is not allowed by the license agreement of the product. In addition to that, there are normally some limitations with OEM versions and some features / content is not included, such as VBA support, content, ... and the printed documentation is also missing.
We have also recently seen some versions labeled as "OEM" to actually be pirated versions. A genuine OEM version will allow you to install service packs, but this is most often not the case with a pirated version. In summary, if the deal is to good to be true compared to the price recommended by Corel on www.corel.com for a full version (there are no OEM upgrade versions), then I would recommend high caution.
Gérard