Perhaps you should try some spelling variations:
ban hang » ngan hang
so sach » so sanh, co sach, hoc sach
bo sach » co sach, hoc sach
hieu va » lieu va, hieu qua
cac tuc » cac toc, cac tu, cac tac
cua tre » cua te, cuu tre, cua triet
y nang » ky nang, ly nang, v nang
nang o » nang co, nang ho, nang ro
o hoi » co hoi, o hoc, o hai

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: ((so sach) or (bo sach)) y nang o hoi. hieu va cac tuc ((ban hanh) OR (ban hang)) dai cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((so sach) OR (bo sach)) y nang o hoi. hieu va cac tuc ((ban hanh) OR (ban hang)) dai cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((so sach) or (bo sach)) y nang o hoi. hieu va cac tuc ((ban hanh) OR (ban hang)) dai cua tre".
• Adding a wildcard symbol may retrieve word variants: ((so sach) or (bo sach)) y nang o hoi. hieu va cac tuc ((ban hanh) OR (ban hang)) dai cua tre*.