Redundant features that make no sense on a paintball gun; like a scope in fixed sights. You can further sell it by claimining you need both for co-witnessing. The weight may ruin performance, however.
Pepper your gun with paintballs and don't clean it off.
Put yellow "rental" grips on your gun, and sloppily engrave a large "serial number" on the side. Tell everyone "it has to be a bad arse gun, that's why paintball fields use them in their fleet."
Freak bore and hone a Tiller barrel.
Remove all factory badging, and tell everyone "I bought this off of Craigslist, the guy said it was a top of the line _______ (insert stereotypical starter gun company)."
I own a disgustingly ugly looking 80's Budweiser cue with a custom made shaft. I enjoy the "wolf in sheep's clothing" routine.