  A feasible, secure and collusion-attack-free quantum sealed-bid auction
protocol is proposed using a modified scheme for multi-party circular quantum
key agreement. In the proposed protocol, the set of all ($n$) bidders is
grouped in to $l$ subsets (sub-circles) in such a way that only the initiator
(who prepares the quantum state to be distributed for a particular round of
communication and acts as the receiver in that round) is a member of all the
subsets (sub-circles) prepared for a particular round, while any other bidder
is part of only a single subset. All $n$ bidders and auctioneer initiate one
round of communication, and each of them prepares $l$ copies of a
$\left(r-1\right)$-partite entangled state (one for each sub-circle), where
$r=\frac{n}{l}+1$. The efficiency and security\textcolor{blue}{{} }of the
proposed protocol are critically analyzed. It is shown that the proposed
protocol is free from the collusion attacks that are possible on the existing
schemes of quantum sealed-bid auction. Further, it is observed that the
security against collusion attack increases with the increase in $l$, but that
reduces the complexity (number of entangled qubits in each entangled state) of
the entangled states to be used and that makes the scheme scalable and
implementable with the available technologies. The additional security and
scalability is shown to arise due to the use of a circular structure in place
of a complete-graph or tree-type structure used earlier.
