There's plenty of loot as you play, and most of it isn't essential to collect D2R Items. While you can sell anything back in town, repeatedly returning to town can become boring and can cost you a little amount in the form of the Scroll of Town Portal. As such, you should spend your time just picking up valuable items.
These are items that are exclusive to an individual class, such as sacred globes and scepters, barbarian helmets, scepters for instance--are a great option, as are amulets, rings, and other items that give an incentive to the skills of the class. You should focus on finding these instead of bolts and other items that take up a lot of space to maximize your earnings and minimize the amount of trips to the town you have to make.
Diablo 2 doesn't have an ending game like you'd expect from an older game, and eventually , you'll probably spend many hours playing boss-runs, where you repeatedly square off against an opponent in order to obtain better items. Even before that point, you may have a feeling that you're weak, so you'll be looking to grind XP (and possibly earn worthwhile items) by repeating certain sections of the game. Tristram, the Countess, Act 1's catacombs, the tombs of Act 2 and Act 3's council etc. are great ground to earn XP more frequently than usual. Be ready to go through these many times.
As mentioned above the ability of the Necromancer of summoning allies can be a great benefit to keep some tension off of you. It's a good thing that everyone has the ability to solicit help even without diving into multiplayer. When you've completed Act 1 story, you'll gain the possibility of hiring your first mercenary from buy diablo 2 resurrected items (one of the NPCs of the Act 1 town, the Rogue Encampment). There are different types of mercenaries to employ as you progress through the game beginning with the bow-wielding rogues of Act 1.