What's a fair price?

This year I sold 20 x 9 cell trays (autumn sown) and 20 x 8 cell trays (winter sown). Same price of £5 for ~20-27 plants. I mostly advertise on Gumtree and Facebook Marketplace. On Gumtree my advert had 102 views and I made 2 sales. On Facebook Marketplace my advert had 324 views, messages from 5 buyers, but only 2 sales (the other 3 just stopped replying   ). Most of the sales have been through word of mouth. 

Every time I list on eBay I get people who live at the other end of the country buying even though I don't offer postage, list it as collection only, and say 'COLLECTION ONLY' in massive letters at the top of the listing description.   They always cancel a few minutes later and I give them a refund. 

The seed was contaminated with vetch again which meant resowing and transplanting.   I did contact Premier Seeds about it but they just said there's an acceptable contamination rate due to how they're grown, something I wish they'd mention when selling the product...

The size difference between November and February sown plants is huge. I think I would sow a bit later than November next time because they got too big for the modules.