Any MacBook built prior to mid 2012 is getting to the stage where a repair to the logic board could easily cost as much as purchasing a replacement computer on Ebay or Gumtree for £100 - £150. However, you can never be sure when you buy second hand whether there is a pre-existing problem like cpu panics when hot or a faulty graphics chip.
These are the pre-retina MacBooks built up till mid 2012
13inch A1278
15inch A1286
The 17inch A1297 is still quite desireable with its massive screen and so its value is more £300 - £400.
I am still happy to repair them and I keep a small stock of parts to help you keep them going into the future. These old MacBooks get a real boost in performance when you upgrade to an SSD.