As the semester starts and you attend Lecture 1 or lecture 0 as some lecturers will call it. They will probably recommend or even exhort(highly recommend) you to buy textbooks.
I found myself buying lots of textbooks in semester 1, only to find that I did not have the time nor energy to pour through all of them. Some textbooks I gave up with them after 2 weeks, others I used them for referencing and nothing more. Having paid good money for them brand new, I therefore ended up with textbooks in almost brand new condition. Even for books that I exercise throughly, I respect them and therefore am able to maintain them in very good condition.
NUS Co-op sells the textbooks you need. Usually the lecturers will notify Co-op about the texts that they are going to use and Co-op will keep them in stock (if the publishers have stock that is). Another place to get textbooks is the Clementi book store. Some say it is cheaper but I have yet to visit the store.
If you are looking for second-hand goods (textbooks), try the IVLE used textbooks forum or the once in a while faculty club used textbooks sale. Personally, I don’t like buying textbooks that I don’t even know the contents because I am unsure if the content is actually useful to supplement the lectures. For arts modules, it is always recommended to buy the texts because you need to do a lot of reading. For use engineering modules, if the lecturer is good, the lectures are usually sufficient. You’ll only really want the textbooks not to recover hidden or uncovered information but as a reference for reinforcing the knowledge and for some practice exercises.
Don’t fret if you really do not have time to do everything. You’ll have to be really clever (in that case you don’t have to do much anyway) or do not require sleep to finish everything. Also, those staying in Hall or having CCA commitments will find it difficult to allocate time to do all your work plus more. I usually stick to prioritising tutorials and lecture notes first before turning to textbooks for information unless the textbook is absolutely necessary.
As for selling the textbooks, post them up on the IVLE forum under the appropriate heading. Include your contact number and for courtesy reasons, your name. Please be truthful about the condition of your textbooks and don’t put 10/10 if you’ve already flipped through it and it now looks anything less than when you purchased it. Usually you can get a good price anyway but the used textbooks forums is for bargains and not a personalised reseller (The cheaper priced textbooks sell fast, since students with deeper pockets would have bought brand new anyway). Some buyers like me value post-its or highlights as it helps focus your search through the textbook. However, I will go so far as to mark pages with removable post-it and not highlights out of respect for books.