I ask you to also look at the rest of the coins. Basically, these are rare or scarce coins, trial coins, coins, rare coins with stamp errors, etc.

This is an authentic original coin. Indefinite warranty from the seller.

Roman Imperial
Constantine II. As Caesar, A.D. 317-337. 
AD 321-324. Siscia.
Obv: CONSTANTINVS IVN NOB C, laureate head right. 
Rev: CAESARVM NOSTRORVM, around, VOT • X in three lines within laurel wreath; 
∈SIS(sunburst) in exergue.
Reference: RIC VII-182.

The coin has passed all the necessary conservation procedures..

Reasonable bargaining is appropriate.