The seller on eBay has this coin listed as being from Trier, with a mintmark of TR. The mintmark is actually [P or S]AR , with the workshop not fully struck due to a weak strike or clogged die. It is not listed in RIC or Ferrando, though Ferrando does have a similar coin (#1266), SAR with L in left field, only missing the ✶ in right field.
Magnentius
A.D. 350- 3
22mm 4.8gm
DN MAGNEN-TIVS PF AVG; bare headed, draped and cuirassed, A behind bust.
GLORIA ROMANORVM; Magnentius on horseback spearing barbarian; L in left field, ✶in right.
In ex. SAR
RIC VIII Arles --
ex NAC 42 #435