The Herefordshire minor counties seamer snapped up five for 35 from seven overs, having already struck a breezy 40 not out earlier.
His innings helped Chepstow to amass a strong 207 for eight from their 40 overs.
Zak Watson (46) top scored for the hosts as they batted solidly before upping the tempo in the later stages.
The wickets were shared among the visiting attack with Jordan Charles and Jonathon Jones picking up two apiece and David Teague, Jonathon Roberts and skipper Paul Swingwood all claiming a single scalp.
In reply, opener Teague (66) played a fluent knock including 11 boundaries, however he was unable to find support as Sellers and Rhodri Fawcett (two for 43 from eight overs) kept the pressure firmly on.
Monmouth soon found themselves in trouble at 120 for six following Teague's demise.
A typically aggressive effort from Roberts (24) provided a late flourish, however the visitors were bowled out with an over to spare for 165. The 42-run loss means Monmouth 1st XI go into their league campaign without a win on the board. The Welshmen travel to Worcester Dominies for their Worcester County League Division Two opener next weekend.