Well, the problem is that yes, the drives are tied to the board - more specifically, the drive board is tied to the main board. Somehow it also deals with authentication of encrypted files. You could swap the drive assembly out (laser, etc) with another - but without the same drive board, you are pretty much SOL.
If you got the unit cheap enough - grab a Blu ray drive, send it into Sony for ~ $150 - they should be able to fix it for you. However I bet that + what you spent will end up being more than the cost of a new PS3.
I'd ask the seller about the drive mainboard though, mention its tied - you don't need the drive, just the board - you may get lucky and be able to grab in, in which case you will essentially end up with a working unit (in time).
For now - use it to view pictures, until something can be done about it (sadly).