We are able to receive payments for stamp "kiloware". But the payment for stamps is not very high. If you are sending in a few stamps with your square to Kat, that's fine, but it is not economical to send a large number of stamps in - the cost of posting would be more than we make from them. :(
Our current dealer does not offer Freepost envelopes unfortunately.
For reference, 150g of stamps is about 1,400 stamps - so it would be ideal if a school/workplace were collecting on your behalf :)
Please trim your stamps down to about 1cm around the stamps. Also, please do not be tempted to take out unfranked stamps for reuse, not only is it illegal(!), stamp dealers are very aware of how many unfranked stamps there are statistically!
Any stamps are OK - domestic, International, normal, plain stamps, or fancy - but only from the last three years please - no ancient stamps!
This is what 150g of stamps looks like, shown on a 13 inch x 13 inch floor tile: