Gmelch, cited above refers to Shelta as a secret language, but doesn't say how secret. She notes that there are Tinkers with very extensive knowledge in judging condition of horses. A tinker seller is a 'blocker', and she explains what they do and who in the trading scheme is called what in slang or Shelta terms.

She doesn't explain one mode that I once read about. The Tinker seller would get another Tinker as bidder to compete with a potential non-tinker buyer. The seller would size up the non-tinker buyer as they went through the process, and use Shelta to signal the 'buyer' Tinker when to raise or hold on a bid, and usually work the price up considerably.