The situation has prompted about 35 voluntary groups to sign a letter calling on the marathon organisers to introduce a fairer system of allocating bonds.
Angela Sherry, fundraising administrator at the Ataxia-Telangiectasia Society, said it had paid £3,000 for a bond for five runners, more than twice the official cost, from an unnamed larger charity. "Our issue is not with the organisation we bought our bonds through but with the marathon," she said. "If it wasn't for this other organisation we wouldn't have had any at all."
Golden bonds were introduced in 1993 and entitle a charity to five entrants for five years. The official price is £1,250.
Only 2,400 bonds exist and holders have the right to renewal. With more than 500 charities on the closed waiting list, many are forced to buy them from other charities.
Sherry said that her charity had been buying bonds from the same charity for several years. With each runner likely to raise more than £1,000, she said, the purchase made financial sense.
Phillip Bush, senior fundraiser at the Royal Blind Society, is co-ordinating the campaign to change the rules. Liberty and SSAFA Forces Help are among those to have pledged support. Bush said he wanted to see the waiting list opened again and suggested charities with hundreds of bonds should be made to give some up.
Race director David Bedford said he wasn't aware of bonds being sold on the black market but described the activity as another form of fundraising.
"We are proud of the golden bond scheme, which has allowed the marathon to become the biggest annual fundraiser event in the world," he added.
This year's London Marathon is on 17 April.