My question is whether it'd be good to get the site set up based on existing classified or matchmaking script/software, with any necessary customisations – and if so, which one people would recommend – or whether it's best to have this programmed from the ground up.

Here a brief description of what the site should do:

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Membership website where members look for offers from other members while offering something to swap.

Members post what they have to offer and at the same time post what they are looking for (wanted) – for which they will be charged.

They can search offers (what a person offers and what they want should be displayed side-by-side) and if they find something they like make a 'swap offer', preferably to someone who's looking for what they have to offer themselves.

The other member can accept or reject the swap offer. If they accept, the contact details are exchanged via the website and all other communication happens directly between the parties.

Each posting, whether 'offer' or 'wanted' has the same parameters, incl. category and geographic location (preferably world wide).

Offers should be searchable by the different attributes that were entered, incl. geographic location.

The site needs to include a way for members to pay for each listing, and for an administrator to be able to

Other than that, the site's supposed to be relatively simple: no member's profiles or ability for members to communicate through the site.

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Hope that's reasonably clear. Thanks for any replies.