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If you mean the thieves guild fences, you will get better ones if you level upin ranks in the guild. If you don't, different merchants have different amounts of gold. Normally the most is 1200 and the lowest 50 (inn keepers). Rohssan in the Market District (A Fighting Chance) for example has 1200 gold.
it depends on your mercantile skill, if it is high the shop owners will have more gold to trade with you, how much the owner likes you may also have play in it.