Beware that many people sell Photoshop at an incredible discount online. These discounts are usually offered because the vendor is selling pirated software. It is pretty silly to pay someone else for sharing something that they got for free.
OEM software is software that is bundled with a hardware purchase. If the online seller is not selling the hardware, then you have a pretty clear sign that it is not a legitimate software license that they offer. Another bad sign is if the vendor says that the software is not supported by Adobe and cannot be upgraded. This is because you cannot ask Adobe for tech support or upgrades on pirated software! A final sign that you may be dealing with a software pirate is if they do not display their physical mailing address online. Most 'OEM' sellers don't want you or law enforcement authorities knocking on their door.
Equal care should be exercised when purchasing a license from an auction site like eBay. An eBay seller should have plenty of positive feedback before you consider doing business with them. The seller should also fully transfer the license to you. The seller cannot simply sell you their old copy of Photoshop 7 if they used it to upgrade to a newer release. You will not be able to upgrade the software if the seller is still using his original license for his own upgrades.
There are more ideal (and legal) ways to obtain Photoshop at little or no cost:
1) Check with your employer and the EULA (end user license agreement) that came with the license of Photoshop used at work. You may be able to borrow the software for your home computer for free. The standard single seat retail license of Photoshop allows the primary user of the software to install the software at home or on a portable computer. You may not use both installations at once. See if your employer considers you a primary user of the software and if they are okay with you borrowing the software to install at home.
2) Use educational credentials to purchase the software at a discount. Try reputable educational software resellers like http://www.journeyed.com or http://www.creationengine.com. Educational credentials might be obtained as easily as taking a class or two at a community college. Educationally licensed software may be used for commercial purposes. One only needs to have educational credentials at the time of purchase. Educational versions are fully functional and may be upgraded to newer retail versions later.
3) If you are a business with several seats, purchase a volume license. There are a few volume license programs described on http://www.adobe.com
Do not trust this FAQ for licensing information. I am not your legal counsel and I am not a representative of Adobe. Always consult the EULA that comes with your software! You can review EULA's for Adobe software by clicking their 'Product license agreements' link at the bottom of each page on http://www.adobe.com.