Yes, you can sell stocks that were bought using DBS cash upfront. However, they will be subjected to the commission charges shown here: https://www.dbs.com.sg/vickers/en/pricing/fee-s...
As for your second question, the answer can be found here: https://blog.seedly.sg/the-ultimate-cheatsheet-...
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DBS Vickers Securities
0.18% to 0.28%
STOCK HOLDING TYPE