1. Take the backup of your current database
2. Run the below queries
3. I have not fully tested that what will be the effect of these queries on the database, but as much i tested it does not have any negative effect on the Magento Database
4. If you find any wrong effect then your comment are welcome
These are the queries to delete the orders on the magento and reset the Id
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SET FOREIGN_KEY_CHECKS=0;

TRUNCATE `sales_flat_order`;
TRUNCATE `sales_flat_order_address`;
TRUNCATE `sales_flat_order_grid`;
TRUNCATE `sales_flat_order_item`;
TRUNCATE `sales_flat_order_payment`;
TRUNCATE `sales_flat_order_status_history`;

TRUNCATE `sales_flat_creditmemo`;
TRUNCATE `sales_flat_creditmemo_comment`;
TRUNCATE `sales_flat_creditmemo_grid`;
TRUNCATE `sales_flat_creditmemo_item`;

TRUNCATE `sales_flat_invoice`;
TRUNCATE `sales_flat_invoice`;
TRUNCATE `sales_flat_invoice_grid`;
TRUNCATE `sales_flat_invoice_item`;

TRUNCATE `sales_flat_quote`;
TRUNCATE `sales_flat_quote_address`;
TRUNCATE `sales_flat_quote_address_item`;
TRUNCATE `sales_flat_quote_item`;
TRUNCATE `sales_flat_quote_item_option`;
TRUNCATE `sales_flat_order_item`;

TRUNCATE `sales_flat_shipment`;
TRUNCATE `sales_flat_shipment_comment`;
TRUNCATE `sales_flat_shipment_grid`;
TRUNCATE `sales_flat_shipment_item`;
TRUNCATE `sales_flat_shipment_item`;
TRUNCATE `sales_flat_shipment_track`;

TRUNCATE `sendfriend_log`;
TRUNCATE `tag`;
TRUNCATE `tag_relation`;
TRUNCATE `tag_summary`;
TRUNCATE `wishlist`;
TRUNCATE `log_quote`;
TRUNCATE `report_event`;

ALTER TABLE `sales_flat_order` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_address` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_grid` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_item` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_payment` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_status_history` AUTO_INCREMENT=0;

ALTER TABLE `sales_flat_creditmemo` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_creditmemo_comment` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_creditmemo_grid` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_creditmemo_item` AUTO_INCREMENT=0;

ALTER TABLE `sales_flat_invoice` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_invoice` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_invoice_grid` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_invoice_item` AUTO_INCREMENT=0;

ALTER TABLE `sales_flat_quote` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_quote_address` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_quote_address_item` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_quote_item` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_quote_item_option` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_order_item` AUTO_INCREMENT=0;

ALTER TABLE `sales_flat_shipment` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_shipment_comment` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_shipment_grid` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_shipment_item` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_shipment_item` AUTO_INCREMENT=0;
ALTER TABLE `sales_flat_shipment_track` AUTO_INCREMENT=0;

ALTER TABLE `sendfriend_log` AUTO_INCREMENT=0;
ALTER TABLE `tag` AUTO_INCREMENT=0;
ALTER TABLE `tag_relation` AUTO_INCREMENT=0;
ALTER TABLE `tag_summary` AUTO_INCREMENT=0;
ALTER TABLE `wishlist` AUTO_INCREMENT=0;
ALTER TABLE `log_quote` AUTO_INCREMENT=0;
ALTER TABLE `report_event` AUTO_INCREMENT=0;

TRUNCATE `eav_entity_store`;
ALTER TABLE  `eav_entity_store` AUTO_INCREMENT=0;

SET FOREIGN_KEY_CHECKS=1;

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