If it is the one for sale in the buy/sell section, my offer was £100. (with either collection or P+P)
When buying something, if the seller asks for offers, simply state what you would be prepared to pay. At the end of the day, the price of something is only what the buyer is willing to pay.
If you think it is worth more than £100 (considering it is 3 years old, no matter how much use it has had) then simply make an offer. If the seller gets a higher bid, go higher (if you think it is worth it.)