I played a little with the assets flags and I noticed some peculiarities:
with pets, although the " REBUYABLE " flag is established, the pets are not actually rebuyable (v.1.19). This does no harm, since the irresponsible pet owner really shouldn’t be allowed to own more and more unfortunate 
pets.
With books, removing “rebuyable” flag does nothing (pages read count), but attention 
: if I remove the " SELLABLE" flag, the book is no longer rebuyable; you CAN sell it though, but it just never appears on the shelf anymore. This peculiarity, I guess, has something to do with multiplicity of and listings in a savefile.
If you ever need to adjust those rebuyable/sellable flags, keep in mind that with some items the role of sellable flag might be to enable reselling, which eliminates the function of rebuyability flag.
If anyone is interested – the idea is, you only gain from reading certain books (like PeopleSkills or confidence boost books) once, at least not the same amount (of confidence, if I stick to the example) as the first time the book was read. With some books, re-reading them (DealingWithStress) brings about the same effect, and with other books (Encyclopedia) we can imagine buying, reading, selling and again buying consequent editions.
Cheers, Phoebe