LIS-SKINS is the option I use most often. The storefront shows the price difference versus Steam, making it easier to find items for a better balance top-up. Buy an item, receive it in your inventory, and sell it on Steam Market.
Best for
Steam top-ups through items.
Finding items with a favorable price difference.
Users willing to wait for a market sale.
What to consider
Compare the price with the actual Steam Market before buying.
Check sales volume, not only the displayed percentage.
Account for the Steam fee and possible waiting time before selling.