ANSWER TAKEN FROM:
When you "buy" ram you stake for it, using eos, which locks it up while you have the ram. You pay a price determined by the system based
on the current % of ram in use (staked for by others). You pay a 1%
eos fee when doing this.
Later, when you unstake your ram, you pay ANOTHER 1% fee, and then
your account is either increased or reduced by the difference in price
from the time you bought.
So if you buy ram early, then more people buy ram, and then you sell
the ram, your eos balance will be higher after you sell... making you
extra eos. This is the speculative nature of ram trading.
If you want to try to play in this market, be careful. Any increase in
RAM capacity by the BPs will decrease the cost of ram, and of course
even the announcment of a side chain being delpoyed by the BPs of the
mainchain will dump the price of ram since that is also an increase in
capacity overall (causing people to unstake their ram on the main
Hope this helps.