For Runescape guides I can see that methods will easily become outdated and therefore people who have purchased said guides will become frustrated because the methods provided are outdated and therefore it is money wasted.
However with that being said I think that guides which might not be related to Runescape directly - but still have implications with Runescape such as malware removal guides (Remote Administration Tools) and the like, would still be useful and would be able to survive for a relatively long time without the methods/guides becoming outdated. (If guides are for sale again DIBs this idea :P )
Not sure what caused the previous selling of guides to be banned however I imagine it was poor quality, badly written guides which were sold as a one off - or easy scamquit method in which case I can see the logic behind it.
To avoid this I think quality checks/controls could be brought in - for example a Guide Verification System could be implemented - part of mod duty or new rank entirely Idk whatever you guys think is best - basically this allows the verifier to check the guide and give it a rating thus allowing for quality control and peace of mind before purchasing.
TL;DR Some guides could still provide decent value and be charged for and quality control should be implemented if this was brought back - however if unwilling or lacking resources to do so current situation should suffice.