Magento – Declaration of … must be compatible with

First of all better to do not mock class under test.

Second one, better to use plugins instead of class overwrite.

And last, you must pass all parameters of parent constructor to your class.

Simplest way to instantiate class is using objectMangerHelper

    $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
    $this->collection = $objectManager->getObject(
        'SR\RewriteSales\Model\Resource\Order\Collection',
        [
            'resource' => $resource, 
            'connection' => $connection,

        ]
    );

Create a new class called Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Grid\Collection (place it in the proper file to match the file path).

The class should look like this

<?php
namespace Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Grid;

use Magento\Framework\Api\SearchCriteriaInterface;
use Magento\Framework\Api\Search\SearchResultInterface;
use Magento\Framework\DB\Adapter\AdapterInterface;
use Magento\Framework\Data\Collection\Db\FetchStrategyInterface;
use Magento\Framework\Data\Collection\EntityFactoryInterface;
use Magento\Framework\Event\ManagerInterface as EventManagerInterface;
use Magento\Framework\Model\ResourceModel\Db\AbstractDb;
use Magento\Framework\Api\Search\AggregationInterface;
use Psr\Log\LoggerInterface;
use Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Collection as OffersCollection;

class Collection extends OffersCollection implements SearchResultInterface
{
    /**
     * Aggregations
     * 
     * @var AggregationInterface
     */
    protected $aggregations;

    /**
     * constructor
     * 
     * @param EntityFactoryInterface $entityFactory
     * @param LoggerInterface $logger
     * @param FetchStrategyInterface $fetchStrategy
     * @param EventManagerInterface $eventManager
     * @param string $mainTable
     * @param string $eventPrefix
     * @param string $eventObject
     * @param string $resourceModel
     * @param AdapterInterface $connection
     * @param AbstractDb $resource
     * @param $model
     */
    public function __construct(
        EntityFactoryInterface $entityFactory,
        LoggerInterface $logger,
        FetchStrategyInterface $fetchStrategy,
        EventManagerInterface $eventManager,
        $mainTable,
        $eventPrefix,
        $eventObject,
        $resourceModel,
        AdapterInterface $connection = null,
        AbstractDb $resource = null,
        $model = 'Magento\Framework\View\Element\UiComponent\DataProvider\Document'
    )
    {
        parent::__construct($entityFactory, $logger, $fetchStrategy, $eventManager, $connection, $resource);
        $this->_eventPrefix = $eventPrefix;
        $this->_eventObject = $eventObject;
        $this->_init($model, $resourceModel);
        $this->setMainTable($mainTable);
    }


    /**
     * @return AggregationInterface
     */
    public function getAggregations()
    {
        return $this->aggregations;
    }

    /**
     * @param AggregationInterface $aggregations
     * @return $this
     */
    public function setAggregations($aggregations)
    {
        $this->aggregations = $aggregations;
    }


    /**
     * Retrieve all ids for collection
     * Backward compatibility with EAV collection
     *
     * @param int $limit
     * @param int $offset
     * @return array
     */
    public function getAllIds($limit = null, $offset = null)
    {
        return $this->getConnection()->fetchCol($this->_getAllIdsSelect($limit, $offset), $this->_bindParams);
    }

    /**
     * Get search criteria.
     *
     * @return \Magento\Framework\Api\SearchCriteriaInterface|null
     */
    public function getSearchCriteria()
    {
        return null;
    }

    /**
     * Set search criteria.
     *
     * @param SearchCriteriaInterface $searchCriteria
     * @return $this
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function setSearchCriteria(SearchCriteriaInterface $searchCriteria = null)
    {
        return $this;
    }

    /**
     * Get total count.
     *
     * @return int
     */
    public function getTotalCount()
    {
        return $this->getSize();
    }

    /**
     * Set total count.
     *
     * @param int $totalCount
     * @return $this
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function setTotalCount($totalCount)
    {
        return $this;
    }

    /**
     * Set items list.
     *
     * @param \Magento\Framework\Api\ExtensibleDataInterface[] $items
     * @return $this
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function setItems(array $items = null)
    {
        return $this;
    }
}

then replace in di.xml this

<item name="offers_ui_listing_data_source" xsi:type="string">Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Collection</item>

with this

<item name="offers_ui_listing_data_source" xsi:type="string">Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Grid\Collection</item>

and this line

<type name="Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Collection"> 

with

<type name="Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Grid\Collection">

For this last line, if your Offers collection expects the parameters mainTable and others, instead of changing the line, duplicate the section that starts with <type name="Wundercarparts\Makeanoffer\Model\ResourceModel\Offers\Collection"> and then replace the line for the one you just cloned.