We do not sell anything that boosts a player's strength. The only things we sell are visual changes (custom looks for your weapon or armor) or convenience features (VIP status which lets you access warhouses from anywhere, notifies when a rare item drops, lets you teleport to cities instantly, etc)
Allowing players to pay for in-game strength would be very unwise as it would chase away large amounts of players who might otherwise join.
The obvious workaround though is that you could offer other players jewels for their items that they have created (by hunting monsters, completing quests, trading, mining, etc). This bypasses the issue of selling strength because we do not generate or directly sell the items. As per our initial timeline, once the jewels gambling games are online it will be a high priority for us to launch our own jewels auction house to simplify the process of selling items between players.