Donate means giving it out free, with no expectations of
returns.
Selling then is expected to get returns.
A person can donate as many times, but limited to three live
births. Meaning after three babies. No more.
But Singapore has lack of donars, so that limit is kinda
sometimes stretched.
SGH Care Centre accepts donations of sperms and eggs, and they
give a token sum of $150/- per successful donation.
Other hospitals, I am not sure they give token sums.
Do take note, that the token sum is only to help deflect some of
your transport costs. And you need to make a couple of trips
to the centre to have yourself checked, blood tested and so on.