Break up multiple associations to the same Model by letting the domain lead the way...

models.rb

# ====== A naïve model =======

class User < ActiveRecord::Base
  has_many :widgets
  has_many :purchased_widgets, :class_name => 'Widget'
end

class Widget < ActiveRecord::Base
  belongs_to :seller, :class_name => 'User', :foreign_key => 'seller_id'
  belongs_to :buyer, :class_name => 'User', :foreign_key => 'buyer_id'
end

# ====== Introduce a Purchase model in between =======

class User < ActiveRecord::Base
  has_many :widgets, :foreign_key => 'seller_id'
  has_many :purchases, :foreign_key => 'buyer_id'
end

class Purchase < ActiveRecord::Base
  belongs_to :buyer, :class_name => 'User'
  has_one :widget
end

class Widget < ActiveRecord::Base
  belongs_to :seller, :class_name => 'User'
  belongs_to :purchase
end