It seems I can't sell it when it's fully modified. It just doesn't show up in my list of weapons on the vendor screen. Is it just me? I don't see this bug on the wiki.

Edit: While we're at it though, which is a more viable sidearm, this or the police pistol? My guy is an explosives/unarmed build, so low skill guns like these are useful for keeping long range critters at bay. It seems like the police pistol does more damage but isn't silenced, and lacks vertical tolerance in VATS.

It seems I couldn't sell it because I was trying to sell it to Joshua, whom I stole the pistol from. Upon attempting to sell it to Daniel it shows up in the menu.

Well, regardless, is it better or worse than the Police Pistol in a combat situation?

.45 is better under most circumstances. Roughly equal DAM, higher DPS, one more round in the clip, silencer. Police pistol's main advantage is higher crit damage. But if your build is crit-oriented... then you're better off with Lucky or That Gun, and Little Devil is better still.