If Function in GAMS

I've been busy with a model, but I'm uncomfortable about the result because I think GAMS violates a constraint. What I want to tell to GAMS is:

"check demand first -> then check current stocks -> IF there is enough stocks sell from current stocks -> IF there is not enough stocks first buy (produce) then sell."

I think in the model GAMS does not obey any demand (sell), any minimum values and sells everything without buying any.

The model is hereinbelow:

Sets
i  items /s,p,b/
t  time in quarters /1,2,3/

Parameters
price(i)       selling price per unit i per quarter in euros /s 6.34, p 6.46, b 5.93/
inistock(i)    initial stock in units /s 320000, p 296199, b 104208/
cap(i)         capacity limit for each unit /s 400000, p 350000, b 150000/
c              cost of holding 1 unit of i /s 10, p 15, b 12/

Scalars
tcap           total capacity of warehouse /650000/

Variables
stock(i,t)     stock stored at time t
sell(i,t)      stock sold at time t
buy(i,t)       stock bought at time t
cost           total cost

Positive Variables stock,sell,buy

Equations
cst            total cost occurs
stck(i,t)      stock balance of unit i at time t;

cst..         cost=e=sum((i,t),price(i)*(buy(i,t)-sell(i,t))+c(i)*stock(i,t));
stck(i,t)..   stock(i,t)=e=inistock(i)+stock(i,t-1)+buy(i,t)-sell(i,t);
stck.up(i,t)=tcap;

Option LP=Cplex ;

Option optcr=0;

Model TWH The Warehouse Problem / all /;

Solve TWH minimizing cost using lp;

Thank you in advance for your support!