Contact the purchaser as follows:
"TAKE NOTICE that should you fail to collect the items purchased from me through eBay in May 2015 (or make arrangements for their carriage at your own expense) within FOURTEEN DAYS of this notice, I shall offer the items for sale in accordance with the provisions of Section 12 of the Torts (Interference with Goods) Act 1977":
http://www.legislation.gov.uk/ukpga/1977/32
If you then resell the goods you will be obliged to pay the money received (less the costs of sale) to the original purchaser.