i would say first of all change the term interest to mark up to get away from all the finance nazis here
it would be a really good move on your part, taking the risk and all. i dont think anyone would say no to an installment program.
16% markup seems excessive, simply because the 21K already includes a certain mark-up. the excessive mark-up should be enough to compensate you for the loss in the value of the rupee and hence, i suggest you use KIBOR, which is around 12.5% these days.
as for recoveries and stuff, get the buyers (or their parents in case of kiddies) to give you post dated cheques for the installment amount for all the installment. its not water tight but its better than word of mouth for sure.
---------- Post added at 10:56 PM ---------- Previous post was at 10:49 PM ----------
P.S. to the people fighting over halal and haram. ARE YOU FOR REAL??? its MARKUP, his profit on a good he is selling. please go research before issuing fatwas. jshak has an item for sale and he is free to calculate the price he wants to charge. Islam does not define a GP ratio above which trades are considered haram ffs