Most merchants in Skyrim operate with chump change. Yes, the
Investor perk can marginally increase that, but the extra 500 G won't help all that much.
The richest vendors in Skyrim are the Fences of the Thieves' Guild. Without help from your Speech skill, a Fence can have 4000 G. When you join the Thieves' Guild you'll quickly be introduced to your first Fence in the Ragged Flagon. Shortly into the main Thieves' Guild quest line, you'll gain access to a 2nd Fence outside of Whiterun. If you've been working "small jobs" for Delvin & Vex, you will have had the chance to recruit a Fence in every other walled city in Skyrim plus one in each Khajiit caravan. When the Thieves' Guild is at full strength, you should have 9 Fences at your disposal.
Wealthier Blacksmiths will generally come in 2nd place with up to 1000G.
If you've got the
Master Trader perk, you can add another 1000G to the starting purse of "participating locations". As such, a "Master Trader" who "Invests" in a Prosperous Blacksmith, can expect them to have a whopping 2500G. Save the Perks & stick with the Fences, unless you're playing a "virtuous" character who won't sink to joining the Thieves' Guild.
Alternatively If you come to the shop with a heavy purse you can purchase some of the vendor's items to add a bit of weight to his/her purse in preparation for that valuable item you wish to unload. Another variation of this trick is to sell to a merchant/trainer after you've purchased training.
FWIW: I'm fond of Delvin's "Fishing" jobs. Nothing like a good excuse to exercise that skill & improve things for the Guild...
Finally, How do you come by an Exquisite Sapphire without knowing about Fences?