In general, I think the rarity of the item can be used to help with deciding how gear should be handled. However, I don't recall at this precise moment of rarity is covered in the Beginner Box rule book. But, basically, very simple and basic things are likely to be within reasonable grasp on most planets (stim packs, a backpack, macrobinoculars, etc..) and can likely be glossed over. Depending on the weapon, armor or gear it could be increasingly more difficult to get it. Stuff that requires black market purchases it would be very reasonable to build an adventure around them trying to get into contact with a supplier or having to do a favor for the fence before they will allow them to purchase what they want. It can also depend on other situational factors, the group is temporarily stranded on some back water town on Tattooine then medical supplies might be hard to come by and replenishing those stim packs could require a bit more work on their part.
Overall though, I'd say hand wave the little purchases for standard consumable items at the least, make them work for the bigger stuff. Then, when they get that customized carbine they've been wanting it means something to them.