Available approaches for moving userpoints from one user to another
There are at least 2 approaches you can choose from, as further detailed below ...

User points donations

Use the Userpoints Donation module, which is one of the modules included in the (D7 version of the) Userpoints Contrib module.

To use it, make sure to also grant the required permission related to it (and to avoid asking yourself "How is the module Userpoints Donation intended to work?" ...).

Rules

Use the integration between the User Points module and the Rules module to "roll your own cookies", by writing some custom rules to transfer user points from one account to another.

As a sample, have a look at this rule, which is used in combination with voting of a node, whereas the author looses "-2" points, and the voter looses "-1" points:

{ "rules_votedown_question" : {
    "LABEL" : "votedown_question",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "answers" ],
    "REQUIRES" : [ "rules", "voting_rules", "userpoints_rules" ],
    "ON" : { "voting_rules_insert_node" : [] },
    "IF" : [
      { "node_is_of_type" : {
          "node" : [ "node" ],
          "type" : { "value" : { "answers_question" : "answers_question" } }
        }
      },
      { "voting_rules_condition_check_vote_value" : { "vote" : [ "vote" ], "value" : "-1" } }
    ],
    "DO" : [
      { "userpoints_action_grant_points" : {
          "user" : [ "node:author" ],
          "points" : "-2",
          "tid" : "0",
          "entity" : [ "" ],
          "description" : "Receive downvote to [node:type-name] \u0022[node:title]\u0022",
          "operation" : "QuestionDownvoteReceived",
          "reference" : "By uid=[vote:uid], for [node:type-name] to q=\u003Ca href=\u0022\/node\/[node:nid]\u0022\u003E[node:title]\u003C\/a\u003E",
          "display" : "1",
          "moderate" : "default"
        }
      },
      { "userpoints_action_grant_points" : {
          "user" : [ "vote:user" ],
          "points" : "-1",
          "tid" : "0",
          "entity" : [ "" ],
          "description" : "Issue downvote to [node:type-name] \u0022[node:title]\u0022",
          "operation" : "QuestionDownvoteIssued",
          "reference" : "From uid=[node:author:uid], for [node:type-name] to q=\u003Ca href=\u0022\/node\/[node:nid]\u0022\u003E[node:title]\u003C\/a\u003E",
          "display" : "1",
          "moderate" : "default"
        }
      }
    ]
  }
}


If you simply adapt the amount of points (-2 and -1) to something like (eg) +25 and -25, you've a prototype to transfer (25) user points from one user to another.

If you replace the Rules event by an event like "something is flagged" (created with the Flag module), a simple click could be used to indicate "I want to transfer 25 user points of mine to this user). You could take this even a step further: since flags are fieldable, you could add a field to indicate the amount of points to be "moved".

However, the above Rules example may need further refinements, e.g. if you want to ensure that after moving points van User A to User B, the remaining points for User A is not negative. Refer to "How to check whether one&#39;s current User Points are negative with the Rules module?" for details about how you can implement such refinement also.