Assuming you’re talking about Skyrim, pre-patch there was a speech check with the elf that works in the brewery that you could do indefinitely. Aside from that, it’s best to sell items individually instead of in stacks as it levels per transaction, not per item.
If you’re asking about Oblivion, I would save up some money and play the wheel game thing, then bribe them when their disposition got too low.
Again, which game are you talking about? If Skyrim, sell everything you can to raise your speech level until you can take the Bribery perk at Speech level 30. Once you've done that, bribe every guard that catches you committing a crime. You'll gain a full level and then some for every guard you bribe. Just remember that once bribed a guard cannot be bribed again for two in-game days.