It is a messy situation, hard to say whether it is lawful.
Certainly an individual councillor cannot make a decision. So the decision to make a purchase is the councillor's own. There is no obligation on the council to reimburse the councillor at this point, since the council did not make the decision to make the purchase.
So the councillor is effectively making a proposal to the council, that the council should agree to the purchase and thus become liable to pay for it, in practice reimbursing the councillor.
The situation would be quite different for the clerk, who can make delegated decisions, so long as they are within the scope granted by the council.
The conclusion seems to be that the procedure may be construed as lawful, but is certainly poor practice.