Can you sell your guns in dead space after you maxed them out to get the maxed out achievement

To get the 'Maxed Out' trophy/achievement. You need to fully upgraded ALL weapons and equipment. To upgrade a weapons or Isaac's RIG, you will need power nodes which are scattered around the Ishimura on the walls or dropped by enemies. You can also buy them at the Store for 10,000 credits. Then you will need to go to an upgrade bench.
When you have upgraded EVERY weapons and ALL of Isaac's equipment you will get the trophy/achievement.

You shouldn't fully upgrade a weapon and then sell it and upgrade another. Note: it is impossible to do this on a single play through. You will need to start a 'New Game +' which will carry over your equipment and weapons from your previous play through.